Why did Paul McCartney play bass?

"Nobody wanted to play bass in those days."

Before the 1960s playing bass guitar was the least glamorous band role. John, George and Paul all saw themselves as guitar players. Then John Lennon persuaded his friend Stuart Sutcliffe to learn to play bass, as a way into the band. Lennon assured his fellow art student that the instrument was 'easy'. 

This was the conventional wisdom of the time but Sutcliffe disproved it  He looked the part but struggled to master the basics - being an artist but not a musician. He even played with his back to the audience to mask his discomfort. 

This created tension within the group and a new relationship helped Stuart to come to a decision. According to Paul McCartney:

“When we were in Hamburg, Stuart fell in love with a local girl called Astrid and decided he was leaving the group."

This downplays the personal animosity between Suctliffe and McCartney, caused by a combination of jealousy and musical frustration on McCartney's part. His goading would eventually lead to an onstage fist fight. 

Stuart left soon after, staying behind in Hamburg. The Beatles returned to Liverpool with a serious practical problem to deal with.

Bass Player Required

So we were now without a bass player. We couldn’t have three guitars and no bass. Nobody wanted to be the bass player in those days because it was always the fat guy playing bass. There seemed to be some sort of stigma attached to it. Paul McCartney; The Lyrics: 1956 to the Present.

McCartney had no desire to switch from six strings to four but  may have felt some responsibility for Suctliffe's departure. He reluctantly agreed to take on the role.

It proved an inspired decision. Paul had largely taught the bass lines to Stuart and was much more competent in applying them. He also had the musical flexibility to start innovating.

He would go on to revolutionise the role of the bass in popular music.