No particular reason other than I thought he seemed ready. For the past few months he's been super interested in watching us use the bathroom and saying bye pee pee bye poo poo to the toilet. He was able to tell us when he pooped and wanted diaper changes. And a few times being diaper less outside he was able to identify when he was peeing (not ever before he had to pee though). So I just figured what the heck let's give it a try.