Servers, the bad ones usually, have this weird sense of entitlement when it comes to tips. Most of them don't seem to realize the reason
they are being tipped, which is to provide awesome service to their customers. I've worked with plenty of them; I know the ones that bitch, and I know the ones that don't.
The only time that bad tips really suck for a server is when you know you provided great service. Rather than thinking, "Man, those customers were assholes," it leaves you wondering, "What did I do wrong?" It's disappointing more than anything.