The Real Reasons People Flirt,

Some people are born flirts — they just can't help themselves.
If they're waiting to get coffee, they'll flirt with the person standing behind them, the barista, and even the delivery guy.
It's just the flirter's natural way of dealing with the world. If they like people and want nothing in return, then they might be more of a charmer than a flirter.
Some flirters have motivations that aren't simply to be friendly and kind; their motives are for gain, and to convince someone into giving them what they want. They're master manipulators.
In his 2004 review of literature on flirting, Northern Illinois University Professor, David Dryden Henningsen, identified six different kinds of motivation for flirting.
The 6 Real Reasons People Flirt, According To A Psychologist