Why do auto parts stores always try to sell you stuff you don't need?

Then they say you need something you dont.Why do auto parts stores always try to sell you stuff you don't need?
I don't have that problem.First, I make it a point to try to get to know the folks working at the Parts Store.The vast majority don't make a commission on what they sell.They honestly want you to have what you need to complete the job.They don't want you to have to make an extra trip back to the parts store, right in the middle of a job.They're paid by the Hour, not by how much they sell.Second, I try to have a Haynes Repair Manual for whatever I'm working on, so I know exactly what I need.For the most part, they really don't want to rip you off, because if enough people feel that they've been screwed over, they'll take their business elsewhere.Why do auto parts stores always try to sell you stuff you don't need?
Never happens to me. I tell them what I require, and if they ask, I say no.