1) Brian Epstein (the Beatles second manager) didn't like his drumming and asked the rest of the group to let him go
2) Pete wasn't that close to the rest of the group and thus made John, Paul and George feel uncomfortable around him.
and perhaps the most debatable reason of all...
3) John, Paul and George were jealous of Pete's good looks and were upset that he received more attention and praise from girls than the rest of the group.
Whatever the reason was, Pete Best was replaced by Ringo just as Brian Epstein began to transform the band into a sensation. It was reported that Pete tried to commit suicide years after he was fired, but a close relative managed to stop him. I do sympathize with him though because after all, how would you feel if you almost became one of the most famous musicians in the world?
YOU SUCK, PETE