It's because when he dies his passive restores him to full health. It's like a 3000 HP self-heal if he's late-ish game. The same passive also gives him 100% lifesteal and 10% max HP damage on hit, so that doesn't hurt either.
And you know what else? The worse you do with him, the more you heal! https://media.giphy.com/media/l0MYIZBbmVeg1iUj6/giphy.gif