Why Lennon and McCartney lyrics changed?

I am like a chameleon, influenced by whatever is going on. If Elvis can do it, I can do it. If the Everly Brothers can do it, me and Paul can. Same with Dylan. John Lennon

I could feel myself climbing a spiral walkway as I was talking to Dylan. I felt like I was figuring it all out, the meaning of life.” Paul McCartney

In their early lyrics, Lennon and McCartney broadly stuck to the Tin Pan Alley lyrical format: essentially I love you/please love me too. 

This allowed for some quirky takes: Paul's When I'm Sixty Four, for example, took a very unusual perspective for a teenage songwriter. But at some level the words usually circled back to the trials and tribulations of romantic love: I Want to Hold Your Hand, From Me to You etc.

Along Came Bob

Two factors disrupted this approach. One was practical - there are only so many ways to say 'I love You' and The Beatles success demanded the rapid production of new material.

The other was their desire to keep ahead of their peers - the driving force of their creativity. By1965 a rival was challenging for the leadership of Cool Club: Bob Dylan.