Science Fact: If you think someone is attractive, you behave in a way that is also attractive, which makes the other person feel more attractive, too.
Explanation: There are some responses almost everyone will experience when confronted by a really attractive person—your heart rate increases, your pupils dilate, your breath quickens, and you feel that butterflies-in-the-stomach sensation. Of course, these are also the exact same symptoms you would experience if you fell down a manhole after having eaten a butterfly sandwich. But some responses—the behavioral ones, not the physical ones—differ between people. A sleazy dude with tons of pickup experience might transition immediately into Slimy Player Mode. "Heyyyy," he'll say, sleazing up to the his target. "Are you an angel from Tennessee in space pants?" he'll inquire, oozing his arm over her shoulder. "Bluurrgh," he will conclude, congealing into a pile of goo on the ground.
Conversely, a shy derpy guy who owns one or more wizard statues might wrack his brain for ways to even approach a beautiful person. "WAT IS GIRL??" his brain would reply, and he would hide in a box until the beautiful person was gone.
So it's a bit of an oversimplification to say that everybody, upon feeling attraction, becomes more attractive. But as long as we keep it together and aren't gross or terrified, liking people tends to make us behave in a more likable way.
The Science: This study is a bit convoluted, so bear with us. A group of guys were told they were partaking in a study on communication, and that they had to talk to a woman over the phone. They were given a picture of the woman they were supposedly talking to—either a woman who'd been rated as unattractive, or one who'd been rated as extremely attractive. Meanwhile, separate groups of judges (ones with no photos of either person) listened to either end of the conversation.
The result was a weird kind of synergy. If a guy thought he was talking to a beautiful woman, his tone of voice and the things he said conveyed that he was interested in her, and the woman responded to that interest in kind (with playfulness, or flirtation, or confidence, or whatever). Observer judges figured the woman was funny, socially skilled, and outgoing, and that the man was confident, happy, and comfortable. More to the point, the judges (who could see neither participant) figured that both were physically hot.
If the dude thought he was talking to an unattractive woman, on the other hand, he was just like "blehh, good morning, I guess" and she was like "no it isn't, nothing is good, sigh," and their conversation was so terrible that all of the observer judges died. Well, not really, but they did just assume that both participants were gross and miserable.
This is in keeping with a well-documented phenomenon in which people tend to behave in accordance with what other people expect of them. If you tell a teacher that a few random students are really smart, and then those students take an IQ test, their performance will improve over their old scores; if you tell a teacher that those exact same students are just average, and those students take that exact same test, they won't improve. We behave how other people expect us to behave (or how we think they do, anyway).
So What Should I Do About It?
Well, this was a phone study, so its lessons don't transfer over exactly to real life. People have an easier time being flirty or outgoing when they're not face-to-face with someone who may very well say "HOW DARE YOU, UGH," and shove them out a window. But the basic principle still applies. The study participants were able to make their behavior much more attractive by being open about their mutual interest, and behavior is a huge part of who's hot and who isn't.
Of course, all of these more attractive behaviors that come out when you like someone—cleverness, engagement, wit, playfulness—are only going to show if you actually approach people and talk to them. Liking someone makes you more attractive... if you can put yourself out there and let people see those attractive traits. So in short, wear your heart on your sleeve, and your Earnestness Underwear under your Confidence Pants.