I had no idea what flirting even was until about two years ago (when I was 20), when an online friend of mine explained it to me. For me, the explanation was just stunning. I simply had no idea what it even was before then. I honestly had no idea that people even did things like that.
Prior to this explanation, an attractive girl had in fact been trying to talk to me in a class, shook my hand (saying "nice to meet you, smile"), and seemed to like getting close to me for some unknown reason. I honestly had no clue what it was all about until my online friend explained it to me a couple weeks later. Of course, I unknowingly brushed the girl off so nothing came of it. Thinking back on it, there were other instances too, where girls were flirting with me, but I of course really, really brushed them off (either completely ignoring them, or even asking them why they were talking to me (poor girls, lol)).
There have been some girls who have tried flirting with me since I learned what flirting is, but they were girls who I had no romantic feelings for (they were "not my type").
I am angry that I didn't know what this was. Someone should have explained it to me sooner, or realized that I had a problem. Before getting lost in the crowd at a giant public university, I grew up in a small private fundamentalist school and there were absolutely no real counselors or anything at all. The "counselors" were actually just youth pastors who knew nothing of psychology or anything really (besides their sect of Christianity, and pretending to be teenagers).