15% is low. So, what you’re saying is that you’re always a bad tipper.
There are tons of factors that can make a good server give bad service. A kitchen that’s lagging behind, for example, is totally out of their control. Another table that demands a lot of attention can also throw off service. You’ve clearly never worked in a restaurant.
Most people tip because they’re not heartless. I don’t think their reasoning applies to you.