Why do guys flirt with girls as a joke?

I HATE these 2 guys in my algebra class because they are jerks! They are nice boys when the other is not around but when they are together they just act like a couple of a**es...why?! They flirt with me as a joke and dit hurts my self-esteem and they are IMPOSSIBLE to ignore :( why do guys do this stuff when they know that it hurts girls' self esteems?