He wants to be the best player in the World. He wont be while he plays with Messi. 4 Years ago when he moved to Barça he (and probably most people) expected to Messi to start to decline now, which isnt happening, ,guy is out of this planet. So the only way he can be the Player of the Year is by signing for another team and winning the Champions League.
He also wants out of Barça because they are selling their brazilian players and not bringing new ones (probably getting Coutinho ONLY if Neymar leaves though). Neymar wants to play with his Brazil's national teamates. He wants the WOrld Cup next year, playing with them would help a lot. If Barça had signed Paulinho and Coutinho I am sure Neymar would have stayed.
Cant say he is wrong, getting the double of his sallary, likely to become an idol at PSG, he will be the head of the team, will have great players alongside him... I would do the same. Everyone would.
Btw, about the financial fairplay, the amount of money they will make out of him is going to be huge. I dont think they will break any rule even spending that much.