Leave - He might be burnt-out

When Ambrose returned over the summer, it was refreshing to see him since he had been out of action with an injury for almost half of a year.
He looked better and seemed to be refreshed. The feuds with Ziggler, Strowman and McIntyre were okay, but he seemed destined for more than tag team championships.
R-Truth wanted to 'kill' former WWE writer. More details HERE
Once people seemed to question his motivations after the reformation of the Shield, you could sense that a heel turn was inevitable.
His heel turn did happen, but he wasn't allowed to fully explore the heel nature of his character. Instead, he was turned into a germophobic character who wore Bane's jacket.
Ambrose also was turned off by some of the cornier things he was forced to do as a face like with Mitch the plant.
With such a tumultuous last two years, it's no surprise that Ambrose decided not to renew his contract once it expires in April.
He might simply be burnt-out by wrestling since he's done it since his teen years, but perhaps he is finally had enough of the WWE machine.
The WWE is often the final destination for aspiring pro wrestlers, but once there, it's easy to get lost in the shuffle or lost in the gears of the machine.
Both of those things seem to have happened to Ambrose and his last straw snapped. Had a few things turned out more in his favor in the recent past, he might have kept rolling with the punches, but sometimes, a change of scenery is the best thing for someone.