This just doesn't make sense. He had developed his career to that point and the big money did automatically follow. I'm sure he was confident enough that he could break into the first team and further develop with better players around him/training facilities etc.
A confident 19yr old would not move solely for the cash and there's no reason to suggest he wasn't confident going by what he had done in the previous months. He was probably overly confident if anything rather than showing a lack of belief.