Well, Brian was just a normal man when they started in 1961. He enjoyed every minute of it, even live he smiled. But at one point, I believe it was 1965, he was crying and he didn't feel well. He said to the others, I believe he talked to his brother Dennis first, that he couldn't handle the strain. It was too much for him: wake up early every day, play a show (with lots of screaming girls in the audience), sleeping for only a couple of hours, travelling etc. In other words: touring itself made him exhausted.
He decided to go in the studio creating new songs for their forthcoming albums and Glen Campbell toke his role as bassist and his high vocals. He did it a few months, I believe. After that Bruce Johnston toke his position and he stayed to the very end (well, The Beach Boys are still touring with him and lead singer Mike Love). Bruce can be heard for the first time on California Girls (1965). He became officially a member in 1967, if I remember well.
Brian liked his staying in the studio. He did not feel very comfortable at stage. During the SMiLE-sessions (which was never released), he used drugs together with his favourite songwriter Van Dyke Parks. Brian lost completely control of himself. And it didn't stay with this...
He had a few breakdowns during his career. But he kept writing songs in the studio. That was the place he felt home best. In 1976, Brian became somewhat healthy again and he began to tour with them again, but he would never be the same again. His voice radically changed. And he didn't look great. He was overweight, he had long hair + beard and well, it looked like he didn't enjoyed his place on stage (again). But in 1983 he began to change. He lost weight and he also looked better facial. On Live Aid in 1985 he looked great and he really enjoyed playing with the other boys again.
And BTW, Carl WAS on drugs and alcohol. It was during the Australian tour in 1978 (which is their worst tour for sure). He was fat and drunk at stage. He completely destroyed God Only Knows for example. A shame, but the truth.
Get The American Band DVD, it's a nice documentary about The Beach Boys, but also about the breakdowns of Brian and the sad loss of Dennis Wilson.
John: "It's the one thing I wish I could do - sing."