One of the most frequent relationship dynamics is that of two dudes who don't get along at first, but they learn to work together; fight crime, make some good jokes, or escape from prison. Of course inevitably they part ways after an argument, but realize they can't accomplish their task on their own, and get back together for an epic conclusion. There are a number of sub-genres, namely Buddy Cop and the more recent annoying guy destroys a competent guy's life. Of course besides films it is also a frequent theme in video-games, especially early video-games, where the two player co-op was very common.
First there are the comedic variants of this, with classics like Planes, Trains, and Automobiles with John Candy and Steve Martin standing out as exemplary examples, but there are many others which are still good, but not quite as good. It's actually incredibly easy to identify these types of movies. Look at the poster, does it have two guys, one with a comical expression? Probably a buddy comedy movie, with the same plot twists you've seen a million times, but maybe some decent comedy. Some opt for hyperbole like the Blues Brothers, though that does not follow the standard formula, as there is no conflict between the characters, creating a very different dynamic.
As stated these are simple movies, with a simple purpose, comedy. Even movies like Clerks follow this same dynamic, with two dudes who hang out together eventually having some huge event that tears them apart, but they get together before the finale. Of course in Clerks there is no real plot, so the conclusion is not exactly action packed or truly decisive, but it still works along the same formula. This has resulted in many brilliant comedies, and many lackluster impersonations. This two man dynamic is relatively easy to right and difficult to screw up, but it is the quality of the comedy and writing that makes it a good or bad movie, so that is why there is such a variance.
Now the second very popular variant is the buddy cop movie, something which has died down in recent days, but still has some appeal as shown by recent films like The Other Guys or 21 Jump Street. Classics like Rush Hour where the main proponents of this type of film, and again it uses the easy and entertaining dynamic to create a decent film. Even something like Bad Boys was still a decent watch, because the main actors work well together. Typically these types of films also have explosive finales which area visual treat, again helping to create the basis for a very decent film, albeit nothing truly amazing.
With many of these genres the film-makers are so constrained by convention that they can't make anything truly genre-defining or defying, because often the financiers refuse to do these kind of risky projects, as they know what works and don't know what won't. This is applicable to a number of genres, but some like Horror and Sci-Fi can still put out interesting and inventive films, whereas some others like buddy and Rom-coms really can't. Genres generally constrain instead of enhance, and in many cases categorizations don't help with the best films, like Pulp Fiction or Donnie Darko. Films are an expansive and all-encompassing media, and we need to shy away from these constrictions and embrace free-form film, especially int he lower budget market.