John Lennon and Paul McCartney had lots of creative differences towards the end of the beatles. They all got together for what they thought would be their last album, Abbey Road, where they were all generally happy to just be finishing their last album and get on with their post-beatle lives. This album definitely feels final and has some sort of resolution to it, making it by no means sad. I am not sure what happened after this (pretty sure it was something about a recording label deal), but they were forced back into the studio for another album after this, Let it Be, which was an atrocity of a recording session where everyone was unhappy with each other, in the process making a really sub-par catastrophe of an album. This was caused by peak tension between John and Paul, who wanted to go in essentially opposite directions with the music. Also while the beatles were recording Let it Be, Yoko Ono insisted she be in the recording room at all times which added even more anger/tension between all members. These circumstances led to some awful recording sessions with generally awful recordings. The music they made, while for the majority half-baked, reflected that kinda sad stage of the beatles in an oddly accurate way given the circumstances