They Did It To Themselves

Ash told the Winchester boys that he had seen them in heaven several times before. He explained that the angels needed them for their plans, so they were always resurrected. After being brain-bleached, of course. Neither man asked for details, assuming that they were constantly being murdered; for that Andy was grateful. He didn't want to be the one to tell them that sometimes it wasn't monsters or hateful hunters that ended their far from picture perfect lives.

Sometimes their death was brought about by their own hands.