He had actually quit smoking for about two and a half years before picking the habit back up. He hasn't talked about why he picked it back up. However, it seems that he is dependent enough on it that he can't bear to not be able to smoke when he flies.
Instead of flying commercial, he's taking private jets so he doesn't have to abide by the FAA regulation that bans smoking on aircrafts. Maybe it's the stress of his stratospheric rise to fame that's gotten to him? He's also said about his fame, that the he's been 'raped,' talking about his loss of privacy.