When is being selfish a bad thing?

When you start putting other people in harm's way or hurt them yourself to get to what you want.

When your selfish acts harm someone else, an animal and/or the environment. As long selfishness won't harm anyone else, if it doesn't hurt any other living being (emotional, physical or mental) then I see no harm in being selfish. I think it's OK to think about yourself too. I even would say we MUST think about ourselves once in a while. We shouldn't count ourselves away all the time either, because that's a bad thing too.

When really the need for being selfish is petty and causes undue hurt.