Often times when I go out to eat I think about how that person serving us may be having a bad day, they got yelled at, relationship trouble, etc. yet they still have to put on a smile and be friendly with a bunch of strangers he/she may never see again. I could not do that.
I tip good and even with bad service I don't tip bad, especially if I have reason to believe something may not be the server's fault. The lowest I will usually go is near 10%.