Brian Wilson and Paul McCartney were big fans of each other's music. I believe Paul's on record as saying "Pet Sounds" blew him away when he first heard it.
I agree, I was just commenting on something I read to the efffect that Brian's obsession was rooted in trying to keep up with the Beatles, especially after hearing Sgt. Pepper's.
That, and the fact that his father was a nutjob.