He left us to go to a team that he could be a starter so he could get into Spain's euro squad.
He never could nail a starter spot on the national team, with a lot of competition in those Positions he's best at. the past week with his comments about wanting to come back to Barcelona sometime in the future, it sounds like he has some regret.
I think he thought going to Chelsea and being a starter there it could bolster his chances at nailing down a starter spot for Spain. But as you can see Chelsea shat the bed and sacked Mou and Pedro didn't have a good season over there, although he did pick up form towards the end of the second half of the season.
He did get in Spain's euro squad but Nolito, got the nod over him as a starter and for a good reason. I don't blame him for leaving but yeah him leaving for Chelsea hasn't really worked out as well as he had hoped.