We will likely never know. IT could be as evil as Dolan being pig headed and as innocent as Donnie wanting to bask in the warmth of a good life and maybe cut back on his duties.
Donnie just might have the physical strength to do the job anymore althought his mind is sharp and his experience vast.
He has a big job that is very demanding in many ways. Maybe its just time for him to go on his own terms.
Thats cool. He is a class act and its been great having him over the delusional prophet Isiah.