Lalwendë makes the astute observation that much of PJ's humour seems of the kind to appeal to the 9 to 19 (or is it 99

) male demographic. This would appear to be the audience group which PJ thought would be most interested in a movie of Tolkien's work and so he appears to have catered to their favoured type of bodily humour.
Also worth considering is the role of humour in George Lucas, whose original
Star Wars provided so much of the visual and special effects inspiration for LotR. The original
Star Wars blended humour and adventure in a light-hearted way that was consistent with characterisation and action. I think PJ strove to emulate this use of comedy but in the end was not able to integrate it seemlessly. So we get a sort of cleaned up Rablasian hilarity rather than a witty humour, which was Tolkien's forte.
My memory could fail me, too, but I seem to recall that Sam was not often used for humour in the movie, but that Tolkien did often use Sam to occasion the odd joke or two.