Well, I am vegan now, but a few months ago I was not and one good friend of mine was. I would often get a little annoyed when we went out to eat and he only got vegan food, usually there was some small reason, like we wanted to get two dishes and share. Thanksgiving at his apartment was a bit unusual as well. Also it was really easy to forget he was vegan and what exactly that meant, like we'd go to a restraunt and I'd ask if he wanted to try whatever I had and totally forget it had cheese or something in. There are so many things that have animal products that it's quite easy to forget - chocolate is one of those that's just so just common, you wouldn't think twice about giving it to someone (although everyone I know knows I hate milk chocolate and love dark chocolate
I'm very vocal about that). There were some chips I wanted the other day, but in reading the ingredients I saw they contained milk.. who would think that CHIPS had animal products in them? In my experience being vegetarian is a lot easier to deal with just because it's pretty obvious when something has meat in it or not. That's just my experience, having recently been non-vegan.