I wrote a whole 280-page book about that, so that's a story for a different day. You don't feel like a fully-realized sexual being and therefore don't act like one.
At some point in your life, you got pegged as a smart person.
Now you could be absolutely stunning (in which case you're both smart AND pretty and everyone hates you except for me -- call me, like, immediately), but your identity is still bound up in being The Smart One.
So maybe you dress frumpy and don't pay a lot of attention to your appearance.
So it only makes sense that in the romantic arena, it should work the same way. The more stuff I do, the more accomplishments and awards I have, the more girls (or boys) will like me. Please say I'm right, because I've spent a LOT of time and energy accumulating this mental jewelry, and I'm going to be really bummed if you tell me it's not going to get me laid.