Well, it's the same motivation, isn't it? "All my peers are doing this - I would be a sucker not to do the same". It's that feeling of being taken advantage of that they want to avoid.
Let's be clear, there are three kinds of mindsets here:
"I don't want to be taxed at all" - lots of lobbying and such to keep taxes on the rich low, and lots of loopholes.
"I think people of my bracket should be taxed higher, but I am going to avail myself of every existing loophole and itemization I can" - doesn't want to feel like a sucker, but agrees people in their bracket, themselves included, should pay more.
"I think people of my bracket should be taxed higher, and I'm going to give money away anonymously or similar to put my money where my mouth is, even if others of my bracket are not" - a unicorn.
