Family
Greed had understood that the idea was survival. They would create another philosopher's stone—and not just a small one, a an enormous stone that would power them for a thousand more years to come—but the ideas and implications Father had set for them had Greed wondering at him. Father addressed them as his children, which sounded to Greed like a bizarre pet name he gave them in order to harbor at least some attachment towards them. And not only was he addressed as son, a title which made Greed feel as if Father said it because Greed had some kind of a responsibility towards him, but he was also forced to refer to the other homunculi as his siblings. Calling that ugly-ass monster Envy his brother was something Greed had never even put the effort to try. And Lust was just too hot to be his sister.
But despite all the attempts to make them feel like family—and odd concept for homunculi who are created artificially—Greed felt no attachment towards his siblings. Lust could make good conversation sometimes, but Pride was never around them for he was too good for them. Envy was just fucking annoying. And Greed had barely gotten to know either Sloth or Gluttony before he left. If Father's objective was to pretend to be human with his family, a ridiculously powerful human, he wasn't doing a very convincing job. Neither of them, other than Gluttony, considered each other any more than coworkers.
They weren't family, not even friends. Greed left to actually find himself one. He'd rather actually choose who he wants to be around than have to put up with those guys while smiling and batting his eyes at Father trying to convince him that he enjoys it.