There is a YouTube video in which a pianist sits at his keyboard, explaining and demonstrating the "bones" (chord progression, modulations---aka key changes, melody, counterpoint) of "God Only Knows" as he leads you through the musical construction of what is not only the "best" song on Pet Sounds, but imo one of the best songs ever written. The craft involved in creating that song, the musical knowledge and wisdom required to do so, is far above what most Rock songwriters are in possession of, and apparently far above what some listeners are capable of appreciating, as is the beauty of the song. No offense intended. ;-)
But I completely understand why some don’t care for the Pet Sounds album. It is NOT Rock music, and sounds in a way "quaint". Most Rock music contains at least trace elements of Blues, which gives the music a "hard" edge, which people like their Rock to have. Pet Sounds contains zero Blues, and sounds gentle, soft, very "white". Is it okay to say that? ;-)
One songwriter who DOES appreciate "God Only Knows" is Paul McCartney, who still considers it his all-time favorite song, and Pet Sounds his favorite album.
Once you’ve heard Pet Sounds’ follow-up---the aborted Smile---PS just sounds like Brian Wilson warming up to do Smile. That Smile was not completed in 1967 and released prior to Sgt. pepper---as was intended---is one of Rock ’n’ Roll’s great artistic tragedies. Hope that doesn’t sound too "grand". ;-)