For John Frusciante of the Red Hot Chili Peppers, business had nothing to do with his decision to leave – just the opposite, in fact. The 1991 Blood Sugar Sex Magik album shot the punk-funk band into the mainstream, and ended up selling more than 13 million copies worldwide. But the quartet's newfound rock-star status troubled Frusciante, who abruptly quit in 1992. "Too high, too far, too soon," the guitarist later said, trying to explain his departure. Drugs and mental illness bedevilled him, before he returned to the Chili Peppers in 1997 for a second tour of duty, only to depart again in 2009.
Though the circumstances of Frusciante's departure on both occasions remain murky, what can't be disputed is that the Chili Peppers made their best music while the guitarist was in the band. For many fans, the Chili Peppers’ most beloved albums were made with him in the line-up: Blood Sugar Sex Magik and Californication in 1999.
Original source: http://www.bbc.com/culture/story/20141212-rocks-irreplaceable-guitarists