Why am I bitter ?

I used to be quite a kind person, a time ago. I wonder what made me change, what turned me into the person I am today, cynical, bitter and a critic of everything and everyone.
One of my friends told me that probably I’ve had a miserable childhood thus developing a defense mechanism that allows me to reject everyone by my acid comments and remarks.
The problem is that I’ve had a very nice childhood, a loving family with a modest income, but not so small to leave me wanting so bad for anything to develop any bitterness. At that time the foundation of my happiness was still based on cartoons, football and running outside all day long not matter of how the weather was.
In high school I was not part of the popular gang, but not really knowing their life I’m not resentful, never really feeling the need to join them. Having quite a modest appearance, for as long as I know, I didn’t really expected girls to be jumping on me. Because of this I always looked more for a kind soul than a pretty face. Naturally I’ve been rejected a couple of times, but I got over it. So I guess that’s also not the cause.
The problem at this point is that the nasty comments are just popping out of my head in each conversation, sometimes being so good that it’s very hard to abstain in saying them. Most of the time they provoke laughter, but occasionally also I hurt somebody’s feelings. However the intent is not driven in this direction, because I say them as jokes, expecting the people around to laugh. To make the matters worse, I usually stretch the boundaries of tolerance to the limit and beyond, very near to the cracking point. Some crack and some … not
So being around me is a funny torture for the others, but for me sometimes it’s also annoying, because no one knows when I’m speaking for real and when I’m kidding.
Ok, I think I’ve cried enough, so for the next blog I’ll write something more me, a great aberration of some sort which I hope will make you smile a bit :D.