Should I be happy for my friend?

So basically... my group of friends kinda became friends with my cousins friends cause we’re the same age. My best friend started dating my cousin which I was totally ok with but then they broke up. My cousin really hurt my best friend cause he dumped her but they stayed friends which is good for me. But then as soon as she got over him they started hooking up again and now they’re like friends with benefits and for some reason it really annoys me. I’ve always been supportive of her getting with him and tried to give good advice but now I just hate whenever she tells me they hooked up! She says she doesn’t like him and that she isn’t gonna get with him anymore but then she always gives in as soon as he makes a move. I know it’s so stupid for me to be irritated by this but I can’t help it! Anyone have any advice?