Why do only desperate guys like me?

I'm a straight female, 18 y/o. I just got out of a bad relationship with a really desperate guy. I find that in most of my romantic encounters I have to "settle" for guys that I'm not particularly attracted to, I don't particularly like, and so forth. Basically, the only guys who ever like me or pursue me only do so because they are very, very desperate. Don't get me wrong - I'm not that special. I don't want to seem arrogant by saying all this. But I'm also not that bad. I've been called pretty by a lot of people, but I dunno if I really believe that. I'm not ugly but I'd say I'm average, I try to offset some of my more unfortunate god-given features by dressing well. As for my personality, I try to be kind to everyone, but I am a little bit socially awkward sometimes which is unfortunate. I'm not really the kind of person that leaves a strong impression on you, I think, but I also don't usually leave a bad one. So once again, neutral.

I find that 90% of the time when I am genuinely interested in a guy, he either doesn't like me back, or he does for a while but then he gets bored of me. The only guys who actually are interested in me only are because they're desperate and will go for anything. Some of them have actually SAID how desperate they are, then hit on me directly afterwards. It really makes you feel bad about yourself after a while... I don't want to date anybody, and I'm not desperate myself. I'm not interested in anyone and that's totally fine with me. But in the past day or two, four of my guy friends have hit on me for this exact reason, 2 of them saying they wanted me because they were desperate. I'm tired of being guilted into giving them a chance only to feel bad about myself later, because I know they're just using me.

Basically... why do you think only desperate guys like me? How do I turn them down while still being kind and keeping them as a friend, and ensuring they don't keep pestering me about it? What can I do to improve my chances of getting a decent guy to be interested in me in the future?