Is Flirting Ever Serious?

I thought I understood

flirting (opens in new tab), but it appears I have no idea how it works. Here are actions that I thought signified flirting from girls:

Guys definitely notice these subtle things—although I've been told:

"You're so stupid, that girl was totally flirting with you and you couldn't even figure it out."

Maybe I'm reading too much into these things. Lately, a girl that I have thought was really cute for a long time has been doing all of the things I mentioned above. Thing is she has a boyfriend. Therefore, everyone around me tells me that I'm just reading into what she's doing. She's friendly and sort of flirty when she's drunk.

I only flirt with girls that I want to eventually hook up with. What is the point of flirting with someone that I don't find attractive or that I don't intend to date? Am I in the minority in thinking this way? Do girls flirt with guys that they don't find attractive?

A lot of girls I know say they flirt just for positive attention.

Bottom line is that it looks like I'm putting too much stock in flirting. Perhaps I'm wrong to give myself credit when a girl flirts with me. Most people seem to think flirting is light-hearted fun, and flirting can be done with no intentions at all. To most people, it is even acceptable to flirt with others when you are in a committed relationship. Maybe flirting is a natural behavior that happens unconsciously from time to time. So, while I think I am not flirting with certain girls—who knows—maybe I am?

Do you have anything to add to my list of flirting behavior? How do you flirt, and do you think I'm putting too much stock in to flirting? Is flirting just common behavior that is so harmless it's ok to flirt with others when you're in a committed relationship? Just because a girl flirts with me, do you think I'm wrong to think there might be romantic interest there?