Giving a tip for a service is expected.
Trent enjoys a meal at nice restaurant and receives the bill.
Trent wants to pay the bill and leave.
Trent pays the bill and includes an acceptable gratuity for his waiter.
Trent pays for the meal but leaves no gratuity for his waiter.
Trent's waiter pockets Trent's tip which he feels satisfied with.
The waiter despises Trent for not leaving him anything which is expected.