I lost the ability to say what I think when I know what I think. I slowly became aware that there are other folks out there who don't like what I say, and I'm naturally very much inclined to making folks happy. I don't like confrontation, and I don't like it when people don't like me.
You can chalk that up to whatever reason you want to, I've gone through several myself and none seem to make a whole lot of sense. I know for a fact that when pushed far enough, I do stand up for myself and so I don't consider myself coward in the traditional sense. But far too often I find myself backing down from everyday fights - arguments, mostly - because I either don't feel it's worth it or because I'm afraid of losing.
The really aggravating thing is that I spend just as much time wondering if everyone else struggles with the same thing rather than just letting it go.