I grew up in a family of tippers (waitress mother and bartender father). EVEN IF someone makes minimum wage (and I doubt housekeepers make more than that), I live by the philosophy of "there but for the grace of God go I", so yes, I will tip if I think someone does a job I would never want to do. And cleaning up someone's room and bathroom after them...no matter how clean and neat you think you are...falls into that category.
We tip delivery guys if they have a difficult time. My husband had a guy come out to fix his treadmill (under warranty, no bill)...guy had a heck of a time. Gave him a tip. I tip my window washer guy (we have a 3 story house, and he charges a ridiculously low amount).
Can't explain it. We don't feel obligated. We feel very lucky for everything we have and truly appreciate those that make our life easier.
YMMV. I'm happy with what I do.