I have a problem; NOT your typical problem! Although it does involve boys and it lies in the friends-with-benefits category (not typical, I tell you!). Hear me out, Jono, because this is where it gets complicated. I've been in a couple FWB relationships before (which were all secret), but I stopped because I decided that they were not for me. But lately, every time I find myself talking to a new guy (from a new school, a new team, etc who has no knowledge of these other relationships), the only thing he wants to do is be friends with benefits. So my question is, am I doing something wrong here? Because I dress quite conservatively and I don't wear too much makeup. Could there be a particular reason that every boy I come across seems to only want hardcore makeout sessions from me? Does it have something to do with my attitude? Am I just doomed to walk the earth without someone who actually likes me for me?
Not a storage shed
Yes, you are doing something terribly wrong! That thing is being 17-22 years old.
Okay, I'm mostly kidding. But this is the age at which guys get romantically adventurous—most of them first do the HND somewhere in this range, or at least start having serious physical relationships. So a lot of guys are going from "I will hold this girl's hand, and maybe pat her on the head, like a dog!" to experiences that are significantly less PG than that. At that point, they're like "Ohhh, okay. Well now I want to do that again and constantly." This attitude doesn't last forever (at least among guys who are remotely mature), but I'm convinced it's partially to blame for your situation. Of course, there's not much you can actually do about being 17-22 unless you have access to wizardry, so the rest of this post will deal with other causes that are more specific to you.
For the record, though, I'm surprised you're getting approaches like this so often. I can picture two friends falling into a FWB relationship sort of accidentally, and then just shrugging and making out, but how do new guys come up to you and propose this? "I would like to make out with you but also with everyone else?" "Are you from Tennessee, because I want to kiss you but not have to talk to you?" There aren't many acceptable ways to ask a girl for a non-exclusive physical relationship. Anyway, here are some other potential reasons for your situation.
1.) You are fun and easygoing.
You asked if personality could be a factor here, and I think it's probably the biggest one. Well, no, the biggest factor is that if a cute girl tells an 18-year-old guy that she'll make out with him if he shoots a stranger with a bazooka, he will whip out his phone and Google "where can i buy a bazooka immediately plz." So the problem might be a combination of these things—their one-track minds and the appearance that you might be down with the idea. That doesn't necessarily mean that you're flirty or confident or whatever—it might just mean you seem laid-back enough not to be horrified by the suggestion. Obviously there's nothing wrong with being fun and easygoing—everyone loves fun people, and easy is the best kind of going you can be—but this could explain why guys are approaching you this way.
2.) You are attractive.
This may not be the most penetrating insight I've ever made, but I think hot people probably get propositioned like this more often. Much like being fun and easygoing, this isn't a quality you'd want to change, or indeed even could, unless you decide to wear a bag over your head or a fake mustache or something. You're right that there are some physical signs that tell a guy you're receptive to being approached—lower back tattoos, for example—but on top of that, if you're attractive enough to attract the jerks, they are going to approach you, jerkily.
I feel like I'm inadvertently painting all dudes as mindless sleazeballs who turn into uncontrollable horndogs, causing me to mix metaphors, at the mere sight of a hot girl. I don't mean to give you that impression. Obviously, guys are not solely driven by physical attraction. But the ones who just want you for a physical relationship are more likely to approach you if you're an attractive person.
3.) You are terribly unlucky.
Of course, we can't rule out that you just had a bad run of unusually horny dudes. There's not much to explain about this one, except that you shouldn't lose faith in the whole gender—most dudes are not like this, but you tend to meet the ones who are, because they actually talk to you instead of getting so flustered by you that they walk into a telephone pole and go to the emergency room. If you find yourself only meeting sleazy dudes, it's because they're usually the cool, confident ones—they don't just approach you more often, they also stand out more.
This dovetails nicely with the "what should I do" part of this post: if this keeps happening to you, take more control over your dating situation. If all the dudes who approach you see you as a makeout receptacle, then make an effort to meet and talk to more guys, instead of waiting to be approached. I'm not saying that you should pick out some mopey saddo whose only virtue is that he is too timid to approach you, just that you should consider some of the guys you do still like but who are a little less brazen. I'm not sure that will even be necessary, though; I think you've just won the Terrible Guys Lottery lately, and soon enough you'll go back to meeting dudes who are actually willing to say words to you and remember your name and stuff.