Fighting. All his life had been fighting. Fighting for his family, fighting with his family, fighting for his life and even fighting after his death: in all, over 400 years of near-constant fighting. One would expect it to get tiring, but Envy never saw it that way. Fighting meant existing, it meant not being nothing. Fighting meant he left an impression (more often literal than figurative) on whatever he fought with, and no matter how long it had been since his human heart had stopped beating, he felt alive.
Perhaps that was what drew him to the shrimp- the hot-blooded son of Hohenheim. It pleased him to see someone else as enraptured with fighting as he was, and he loved the power struggle that ensued every time Envy shoved his tongue in his chibi's mouth and put his hands where they didn't rightfully belong. Each night he made Edward his by sweat and blood as they did battle, tangled in sheets, and every time Edo moaned his name felt like a glorified accomplishment.
When he thought about how the shrimp made him feel, Envy couldn't believe how close he must have come to being Lust. But he had Ed, had fought tooth and nail for possession of him, and would fight twice as hard to keep him. Envy would have him forever, and that was almost enough to bring him peace.