All the film descriptions I've read say John and Max have been feuding since childhood. Yet until John and Aerial start their relationship, they seem to get on just fine. Max even has an opportunity to turn John in to the IRS, yet doesn't.
In any event, why would two people who hate each other choose to live next door?
Yeah, they don't seem very grumpy at the outset either... The title led me to expect a different type of film, but I'm still pretty happy with this one, because Lemmon and Matthau are old hands.
Born when she kissed me, died when she left me, lived whilst she loved me
They were childhood friends who grew up in the same neighborhood.
Their feud began when they were adults and wanted to marry the same woman; a woman who John eventually married. As it turns out, Max ends up marrying a woman who is a better wife. These facts come out during an argument between Max and John.
The conflict reaches new heights when they both become interested in Ariel. That is why Max accuses John of "...doing it again!"