Rules differ from place to place, but where I am, a lot of people just say "This name fits me better," or something to that effect. At my hearing, that was the reason given by both the ftm buddy of mine who happened to have the same court date, and also by some random cis guy. Another guy was there with his family to change their last name. The reason he gave was that their mail went astray a lot because so many other people in their apartment building were also named Ali. The judge thought that was a perfectly good reason. As long as they don't think you're trying to defraud anyone, they're happy with pretty much any reason you give them.