Science Fact: Men are more likely than women to think their opposite-sex friends are romantically interested in them, and are also more likely to be interested in their opposite-sex friends.
Explanation: You've probably had a close friend of the opposite sex. Maybe you were so close, someone once implied there was a romantic connection between the two of you. "Who, her??" you retorted, sounding shocked. "Ha ha, blech! I would rather kiss a bag of slugs!" you exclaimed, perhaps protesting a bit too much. "I'm so not interested in her that I would rather die of kissing slugs!" you continued, rolling your eyes and sticking out your tongue for added effect. "I will literally eat slugs right now, to cleanse my palate after the thought of kissing her!" you summarized, following the other person down the hall as he attempted to get away from you.
So yes, of course you were more interested in her than you would care to admit. And maybe one day, you'll be hanging out with her, just being yourself and casually doing something you both love. You'll start noticing stuff about her—the fluidity of her dice rolls, the elegance with which she moves her orkz—and at one point, to your smitten mind, it will totally seem like she is leaning in to kiss you. You will lean in to kiss her back, but it will turn out that all she was doing was maneuvering her weirdboyz, and with your eyes closed you will only succeed in biting her hair. "Ow ow ow why are you pulling my hair??" she will demand, trying to turn her head to look at you. "IFT'S THTUCK IN MUH TEEFTH!" you will try to explain. It will be the most embarrassing disaster in human history, all because you are male and therefore have no idea when a girl does and does not like you.
The Science: Some researchers got together pairs of friends and surveyed them about their friendships (they were sure to bring in both actual people, to make sure the friendships weren't "one-sided," like if one of the friends turned out to be a dog or a ghost). With the pleasantries out of the way, they asked the participants if they secretly wanted to boink their friends; guys were more likely than girls to admit that they did. Also, guys were more likely to think this attraction was a benefit, rather than something that got in the way of the friendship (girls were more likely to feel the opposite way). Also also, guys were more likely to see physical attraction as a reason for initiating opposite-sex friendships in the first place.
The researchers behind the study stressed that none of this was too unusual—guys are pretty much constantly overestimating women's romantic interest, and if you put a wig on a porpoise, they will probably assume it is asking them out.
So What Should I Do About It?
This is an easy one: learn how to tell if someone's interested. For example, a girl wearing a skirt is not trying to communicate that she likes you more than a girl who is wearing pants is. The only thing a girl wearing a skirt is communicating is that she is wearing a skirt. Associating a woman's attire with her interest is one of the many mistakes dudes are prone to making, when what they should be watching is a girl's behavior and body language. Brush up on your flirtation cues, pay attention to the things the woman is actually doing, and remember that the more you like her, the more likely you are to think that she likes you (even when she doesn't).