If you think their intentions are good, then it's kinda rude to snubb them. Simple as that. It's ok being rude, and it's your life. Not everyone has to be a nice guy.
You don't have to say thank you to the waiter (I don't recommend being a jerk though, they do handle YOUR food), or let merging traffic in either. You can go through life with that attitude... but isn't life so much better if we were all a little friendlier?
I need to hug a tree now and drop some acid.
