I know that I can't say no sometimes. Fear takes over, it's almost like I have a built in defense not to make someone upset with me.
I do that too.. Afraid of upsetting someone or getting in an altercation (only with people I care about though, I just get really nasty if it's someone I don't know, like if i'm out... Then it's the opposite and I want to beat the hell out of them) I even do it at work, if they call to ask if I can come in and work or stay late I always say yes to make them happy.