Why do girls flirt with you if they don't want it to go further?
Because flirting is fun.
Because flirting is fun. It's fun for girls to talk to and tease guys. And it's nice to feel attractive - to think that someone is attracted to you and thinks you're cute or interesting.
If a girl is flirting with you, that doesn't mean she owes you anything. Flirting doesn't mean you've got the right to take it any further. Everyone has the right to decide what they do with their body - how far they will go and what they will do.
Remember too that you and the girl may see things differently: you think she's flirting, but she thinks she's just being friendly.