Why Guys Marry Some Girls (but Not Others)

It happens all the time: A guy spends months, even years, in a long-term relationship with a girl he really digs. But after dropping the I-don't-see-myselfever- getting-married bomb, he suddenly turns around and ties the knot with a new chick. The factors that tip a dude from steady relationship to "till death do us part" seem like the ultimate unsolved mystery ... especially when you're in a solid LTR and aren't sure if your guy is even considering marriage.

It all comes down to some elusive qualities women have a hard time understanding but men are always on the lookout for, explains Willard Harley Jr., Ph.D., author of I Promise You: Preparing for a Marriage That Will Last a Lifetime. "Part of what makes him want to get married is chemistry and passion, but it's also about certain actions and behaviors that are more concrete than you'd think," says Harley. Below, we clue you in to five crucial traits that separate the girls men date from the ones who make them want to set a date.

Tie-the-Knot-Trait 1: She's Exciting and Always Evolving

You know how every season finale of your favorite TV show ends with a million unanswered questions and you can't freaking wait for the next one? Well, a girl can give her guy that same thrilling mix of exhilaration and anticipation by surprising him.

"She does this by being spontaneous and a little unpredictable, taking on new interests all the time, and revealing different facets of herself," explains Alon Gratch, Ph.D., author of If Love Could Think. By never letting life get static, this woman busts the myth that being married means feeling humdrum. "She makes being with her an adventure, as if there's always a new idea or activity just around the bend," says Gratch.

"With most of my girlfriends, I feared that we'd run out of things to talk about if we spent too much time together. But when I met my fiancée, Gwen, I kept discovering new layers to her. One day she came home from work with a stack of cookbooks; the next week she told me stories about how she loved designing outfits in high school. She's full of small surprises." —Brett, 29

"I dated a lot of girls who liked pushing boundaries, but it all seemed a little forced. My wife's sense of adventure, however, comes from within. She's naturally driven to challenge herself by trying new things. That's the kind of girl you want to marry. If you aren't continually fascinated by each other, it won't last." —Rob, 38

"The thing that makes my fiancée so captivating: She takes risks. I don't mean she bungee jumps off bridges. It's more that when a new opportunity comes along — a different facet to her job or the chance to meet new people at an event or party — she grabs it and proceeds. Her boldness makes her enchanting." —Bob, 27

Tie-the-Knot-Trait 2: She Really, Really Loves Sex

No big shocker here — a chick who enjoys twisting the sheets will always have a hopping social life. But when a woman makes the effort to have really connected sex that involves both body and mind, she goes from great girlfriend to marriage material in his view.

"Men crave sex that's erotic, but they also want sex that makes them feel deeply bonded," says Gratch. In other words, it's not all about wowing a guy with pretzel-like positions; a big part of having amazing booty is paying close attention to his mind-set and moods during the deed so sex reaches a higher, almost spiritual level.

Another thing that makes them think of the M word: when a woman is actively committed to keeping the passion on high boil. "A guy's biggest fear is that the great sex that made him think you were The One will fall by the wayside," explains Douglas Weiss, Ph.D., author of The 7 Love Agreements: Decisions You Can Make on Your Own to Strengthen Your Marriage. "A woman who puts effort into maintaining that sizzle is a dream come true."

"To make sure that the awesome sex we had when we first met didn't lose its passion, my wife came up with this idea for regular ‘sex bets.' For example, she'd bet me that I couldn't make her climax several times in a row, or I'd challenge her to initiate action in a semipublic spot. We have never fallen into a rut, and our competition keeps us feeling connected." —Jamie, 30

"My fiancée did this terrific thing when we first started having sex: After we were finished, she'd tell me how good I made her feel, that she really liked how I felt against her skin. It made me want to tell her what I liked too. Guys aren't supposed to admit it, but opening up about how sex affects us emotionally actually enhances the physical side of things." —Paul, 28

"My fiancée was up-front from the start about her desire for an extremely satisfying sex life; she never had any hang-ups about taking charge in bed or proposing out-there activities that a lot of women would be afraid to admit they were curious about. I could tell I wasn't getting ensnared in the classic bait-and-switch marriage that so many guys fall into — you know, when the action slows to a halt a year after you become husband and wife." —Russ, 34

Tie-the-Knot-Trait 3: She Makes It Clear He's Not Her Entire Life

It's flattering to a guy to realize that his girl thinks the world of him, but it's less appealing when he gets the impression that he is her world. That's why a girlfriend who retains her independence and sense of self, even as the relationship takes a serious turn, has definite wife appeal.

"A woman who depends on a man for her sense of fulfillment is a scary thing for a guy," says Gratch. "Men don't want to feel smothered or totally responsible for their partner's day-to-day happiness." The guy ideal: a chick who views coupledom as a solid partnership in which both she and her man still have separate identities.

"Before we were married, Jess would go out with her group of friends a lot, which I later became a part of. But she didn't put all of her focus on me. She made it clear that she was there to hang out with them. I really liked the fact that she wasn't the type of girl who ditches her girlfriends when she meets a guy. It made me confident that she'd always have her own life outside our relationship." —Sam, 33

"She definitely makes time for me so we can do things as boyfriend and girlfriend, but my fiancée also keeps up her own life. She has a weekly dinner with college friends, an art class every Thursday night, plus the responsibilities of her job as a journalist. I like that she doesn't check with me first to see what I want to do and she doesn't offer to bail out of an event or night out with pals in favor of always being with me." —Charles, 35

Tie-the-Knot-Trait 4: ...Yet She Still Conveys How Very Important He Is to Her

Okay, so men dig independent chicks. But there's a fine line between being independent and acting aloof and detached. Guys settle down with a woman who regularly reveals her tender side and shows him that no matter what happens, she's got his back.

"Men view life as a struggle or war that they're fighting, and they want someone who'll be on their side at all times, through thick and thin," says Gratch. "This doesn't mean that a woman should mother him or assist him with every little detail of his life; it's more about consistently doing small, nurturing things that let him know you really care."

A couple of examples: Bringing him a treat when he tells you he isn't feeling well or complimenting his brilliance after he finishes a stressful work project. "These gestures are tiny, but they reassure a man that his girl is solidly on his team," says Gratch.

"When we first started becoming a serious couple, my fiancé accepted a new job that required a lot of travel and attention. Still, even from hotels across the country, she took the time to call and check in on how my day was going and e-mail me little messages. She even stocked my refrigerator with beer and sandwiches before she'd leave on another trip. Her concern and thoughtfulness helped push our relationship to a higher level." —David, 28

"Every so often, my in-laws and I will get into an argument. But from the very first one, my wife has always respectfully defended me, and this made me want to be with her forever. She loves her parents and values their opinions, of course, and she and I don't always see eye to eye. Yet no matter what the issue is, she still lets her family know that she's sticking by me. This is a big thing for guys. I know it sounds ridiculous, but men tend to see themselves as misunderstood lone wolves."-Alan, 30

Tie-the-Knot-Trait 5: She Wants Him to Be the Best Man He Can Be

Men don't secretly want their girlfriends to bark orders at them like a drill sergeant. It's more about helping him reach his potential by actively supporting his goals, even pushing him a teensy bit so he can succeed in whatever he does.

"Young guys tend to try to get away with as little as possible, and a lot of women let them, thinking that it would be out of line to challenge their man," says Harley. "But a man admires a woman who encourages him to shape up and toe the line ... as long as she has his best interests in mind and isn't trying to mold him."

This also means calling his bluff and not letting him get away with slacking off. "Even though it might seem like tough love at the time, she helps him achieve and accomplish things, and deep down he appreciates her for that," says Harley.

"Before we got engaged, the woman who is now my fiancé told me that she thought I was drinking too much, working too hard, and not taking care of myself. Ouch. But when I actually took in what she said, I knew that she was right and she was looking out for me. No other girlfriend had ever been so honest." —Ryan, 29

"I wasn't on speaking terms with my father for years, and when I told my girlfriend this and that I just didn't care about having a relationship with him anymore, she didn't nod sympathetically. She made me call him and work things out because she knew I'd eventually regret it if I didn't. She had the guts to disagree with me about something most women wouldn't want to get involved in." —Shawn, 31