When I was starting out as a young adult and dining on my own, I used to give 15%.  I thought that's what I was supposed to give (this was in the late 90's, early aughts.)  However, a couple of servers who had been perfectly friendly suddenly turned their back on me near the end of my visit.  I wondered if I had offended them.  Dad told me later that 20% was now expected as a tip, not 15%.  (BTW, the restaurants in question were Chili's and Denny's).  Nowadays, I give 20%, even at buffets, because I'm so scared of offending the servers.  I usually also round the tip up to the next dollar.  I'd like to say it's because I'm an Italian-American and we're so generous!  But it's really because I'm scared.