Conservatives tend to believe in individual individual responsibility. They tend to think that all should be left to themselves to prosper or not. Democrats believe in a social contract. We believe that we have a shared responsibility to care for our fellow man. The Democratic philosophy is one that lends itself towards tipping.
Tipping is like karma it comes back to you and you are rewarded in kind. If you get a reputation for being a bad tipper you will get bad service and vice versa.