Here's one of those movies who's screenplay seems to have come from a writer thumbing through "Us," "People" or some other worthless checkout-counter pablum, reading about pop-culture non-words like "man-date" or "man-cave" and going "A-HA!" I'm not sure if that's where "I Love You Man" came from, but that's what it plays-out like. Is it funny? Sure. But much like the non-words forming it's high-concept, nobody will remember it in a year or less.
Paul Rudd is playing a realtor named Peter who's impending marriage has inadvertently sent him into a mini-crisis - amid his consideration of a "best man," his family and friends point out that he doesn't have (has never had, really) any close male friends; certainly not a "best" one. The reasons for this are easily divined: Peter is the Perfect Boyfriend, a one-man girl-drink-mixin', chick-flick-toleratin', problem-listenin' machine who's dedication to pleasing his ladyfriends has left him without a discernable male social life. Now, the poor guy is seeing pairs of Good Buddies everywhere he looks; so he embarks on a quest to "pick up" some Y-chromosomed compatriots.
So, it's the "formula" of a romantic comedy applied to a story of platonic male friendship. There've been worse ideas. The film is at it's weakest (though still amusing) early on as it name-checks the tropes of "man-dates" and expected gags - with the hysterical exception of Thomas Lennon (your go-to-guy for ambiguous homosexuality) as an "ideal" suitor who's notion of "man-date" is significantly more literal than Peter's. The film get's to it's "point" when Peter meets slovenly uber-masculine slacker Sidney (Jason Segel) and they hit it off... to the point that it starts to cause some friction with Peter's regular fiancee-centric life.
Yes, it's another scion of "Clerks" in which a guy's rocky road to adulthood is impeded, commented-on and (maybe) helped by his wackier best bud. But it's reasonably funny, even if it won't likely be remembered as a high point in anyone's career.