Answer: Nothing, probably. The way you describe yourself, you sound like a perfectly normal, healthy introvert. A smart one at that. Add those two and it's not at all unusual that you'd take a little longer to form the kinds of relationships that don't come naturally to you.
Your best option is to accept yourself the way you are, give yourself time, and direct whatever effort you put into this toward getting to know more guys as friends. The more nonthreatening the circumstances, the better - say, in classes where you're in your element, or a summer job, where you have both proximity and relatively equal status with your fellow summer employees.
Why does the "smart" matter? Overthinking, usually (on your part), plus there's the intimidation factor (for others). A girl who sits at the top of the class and sticks to a couple of close friends can be very daunting to approach. Not that this is bad - or is even what's going on with you, necessarily - it's just a common set of circumstances.
I realize I'm assuming the doesn't-come-naturally part in your dealings with boys, but a smart introvert who has, say, a pile of brothers might feel comfortable around boys and have no trouble getting dates. In that case, the frustrating thing for her might be in making close female friends.
Comment: I was a "late bloomer" and didn't have my first kiss until I was 20 even though I was considered one of the "popular kids" of my high school and college. I have had a very active dating life in my adulthood (I am now 28), as well as in my junior and senior years of college.
Many of my friends who had boyfriends in high school and in early college look back at their friendships as the high points, rather than those relationships.
I also credit my late blooming as part of the reason I am self-sufficient and independent, which actually can attract guys (and the right kind of guys, who appreciate you for you).
I guess all I have to say is - don't worry about it. I remember feeling very frustrated and thinking there was something "wrong with me," but in retrospect, I just was focusing more on figuring out myself and enjoying a close group of friends. The boyfriends came along in time.
A: Even if it doesn't play out for her exactly as it did for you, there is no substitute for "focusing more on figuring out myself and enjoying (whatever)" as a path to a happy outcome. Thanks.
E-mail Carolyn Hax at firstname.lastname@example.org, or chat with her online at noon Fridays at www.washingtonpost.com.