Have you ever been volunteered for something, especially something you don't want to do. It's the worse. Why do some people think it's okay to say another person will do something without asking them? I mean, how dumb can you be? People have their own lives going on, they aren't at the beck and call of you. Don't volunteer someone else, do it yourself!
Then, if you are volunteered to do something, you look like the jerk when you can't do it because you are busy with something else, you let someone down; not the idiot who volunteered you. So, you inconvenience yourself and try to get it done all the meanwhile seething in rage (okay, that may be a little extreme, but probably not), not wanting to be doing whatever asinine thing they volunteered you for. Then resenting the person who volunteered you. Doesn't it just make you want to smack them?