There's two different reasons to tip people in my opinion.
The first group I tip well are the people I think are doing a great job and deserve it. Maybe they went above and beyond with service, I could relate to them, or they just have a shitty job in general and look like they could use it. Example of this, the guy who always serves me at the breakfast diner on the weekend and remembers what I want. I appreciate that so even if the meal is like $10 i'll just toss him a $20 and leave. The woman who cleans my apartment once a month or so, and sometimes brings her 2 little daughters because she prolly can't afford a sitter. They are always well dressed, polite, and help their mom, so I'll give them a $20 and tell them hard work gets rewarded. My barber. $12 standard cut, but this guy gets a $20 every time because he's got a straight razor.
The other group are people that I feel like I have to tip, or they can screw me over. Mailman, paperboy, doormen, building handyman, valet driver (tipped upon arrival), etc.
But I don't feel obligated. If a waiter messes up my order and doesn't fill up my water, guy will be lucky to get 10%, I don't care who he has to manditory tip out he should have stepped it up then. If a taxi driver takes me a shitty route because he thinks I'm a tourist, guy is gonna get paid with credit card (they hate that cause it costs more fees) and no tip. Tip is a privilege that you have to work hard to not get, but it's possible.