You've worked in a restaurant before and understand that a little more than what's expected can make somebody's night, or you're terrible at math. After all, figuring out 20% is easier than 15%. You think anyone who needs their iPhone to figure out a tip is a tool.
You're probably someone who worked your way through college as a waiter, and now that you're well off, you know you can make somebody's night. Either that, or the server is very good looking and you think that maybe flashing your wad will buy some love. It won't... unless you are also very good looking.