"The problem with reverse psychology is that I'm quite contrary."

He changes his voice up a bit, obviously doing an impression of someone as he mimics getting scolded. "'Mister Strange, I don't see why you'd do this bit of magic in the first place.' Well if there's no reason why I shouldn't do a bit of magic then of course I'm going to do it!"

It's also 100% apparent that Strange is awful at impressions.