Only at the end, in the wake of a brilliantly executed party sequence — in which Roger, the solitary Gen-Xer, finds his world of defensive ironies and carefully preserved pop cultural references overrun and trashed by a swarm of Millennials — does his arc, as residents of Hollywood might call it, become apparent. Mr. Baumbach abruptly, and with a subtle display of self-conscious wit, reveals “Greenberg” to have been a romantic comedy all along. Here we are in a car speeding toward the airport and what might be the prospect of a happy ending. And suddenly a movie about a man who is defiantly difficult to like becomes very hard not to love.