Pizza delivery isn't going to make anyone rich. Although the pay scale may change somewhat from area to area, many delivery people make less than minimum wage. They may receive a small extra bonus for each stop on their route, but that isn't going to put anyone through college, either. The specifics will vary, but the take away is that drivers rely on tips to survive. You're not obligated to pay a tip, but when you don't, you aren't hurting the pizza parlor, the chain (if it is one), or whoever it is that put you in a bad mood. You're hurting what may be the lowest man on the pizza production totem pole, and the one who can least afford to take the hit.