In general, I don't. I'm not sure why, it's just a personal thing...if I do share, I must make an agreement with the other person beforehand so I know what's expected.

Once I had a friend just reach over to my plate with his fork and in the course of this action he tells me he wants to try my meal. I instinctively grabbed his hand and asked "what the heck are you doing?"

I know it's a great idea to share food (so people get to taste things they may want in the future), but I really don't like it. I'm okay if we "make a deal," but people who think my plate is their plate...it bugs me.