How do you know when a tip is appropriate and when it's unnecessary?

I recently had my windshield replaced, courtesy of my auto insurance. The auto glass guy came to my place of employment and replaced the windshield in the parking garage. I did not tip him, and when I asked my coworkers, they said I should have. I called and am going to mail him a check for tip amount (debating between $10 and $20).

I did not tip him because I was thinking when you bring your car to a mechanic, you don’t tip the mechanic, you just pay the bill.

A few years ago, I had a cable man install cable in my apartment, and I tipped him. A friend of mine told me “you didn’t have to tip him, he was just doing his job.”

I am totally willing to tip when it’s expected and/or necessary. I tip the hair dresser, for example, and I tip wait staff in restaurants. However, I don’t want to tip if the situation does not call for it.

When you read tipping articles, they always describe situations as “a tip is appreciated.” Of course it’s appreciated, my question is “is the tip necessary and expected?” If it’s not necessary and expected, I don’t have money to give out if I don’t have to.

How do you know when a tip is necessary and expected, and when it’s unnecessary?