If you're too harsh or too demanding with your mechanic, he'll throw a screwdriver across the room, cuss you out, and that's it. Or he'll smolder and wait till you leave, pretending to be cooled down, then gouge your radiator before you get back, or cut your tubes.
Don't ever criticize or question a mechanic, unless of you course you question him like a child, meaning you don't know a thing about a thing. They don't mind that so much. It's good for their ego.
The same thing about guys or companies that mow your yard or scoop your snow. They know they have a million other potential customers, they don't need you. So any little demand, phrased improperly or with insistence or with anger, is going to be your final one. Because they'll be outta here.
I knew of a grass cutting place, and now that I think of it they also did snow. You didn't want to say Boo to them. And a guy I knew said more than that to them -- a guy without a diplomatic streak, who obviously thought he could talk to them frankly and without fear. That was it! They're outta here. Cut your own grass! Scoop your own snow!
Snow guys are especially bad -- let's say they're just snow guys and don't cut your grass too. Because when it comes to snow you're a lot more desperate than you are with grass. Plus, when it comes to grass, everything's staggered as far as schedules. It doesn't matter if they cut your grass today or three days from now. Everyone's grass is growing at different rates, etc. But with snow, everyone needs it done now, because it snows the same time for everyone.
So the snow guy is automatically overwhelmed every time there's snow -- they're snowed in, if you'll forgive a little unsolicited humor. So if the snow guy has the slightest tendency toward having a short fuse, you're going to set him off if you make any kind of demand, like "Hey, where are you? Aren't you going to scoop my snow, you crumb?" They know they've got you, so they can pack up their blade and leave.
The only way to handle a snow guy is with the most delicate and refined diplomacy. Get on his good side by commiserating with him about how busy his day's been. "You've really been working hard! Wow!"
Then, when he invariably does a bad job, just hold your tongue, keep your wits about you. Because it will snow again someday and you'll be at their mercy again. When it comes to snow, a half job is about as good as a full job. As long as you can get through.