Is it my fault?

I always go for the nicest guy in the room who makes me laugh and treats me so nice. But after we’ve been together for a while they keep turning into huge assholes and I don’t know why. Is it my fault, am I turning these nice guys into dicks? I know I am a pushover and want to take care of anyone I care about to a point that sometimes even mothers wouldn’t do, I just care too much and I’m working on it. Do I let these guys walk all over me and then they become assholes? Or is it just a coincidence?