Weezer is one of the most beloved bands of the 1990s. They delivered a universally adored classic with their 1994 debut, and a 1996 sophomore effort that has risen to the status of cult classic over the years. Their subsequent output has been successful, but derided for lacking the quality of these first two albums.
The difference between Weezer 1.0 and Weezer 2.0 (or whatever number we are on now) is one man. Well, at least, it lined up that way. Matt Sharp was the bassist for the band during their 90s peak, and then he departed to work with other bands and artists. What the heck happened?
Matt Sharp left Weezer in 1998

Sharp, a founding Weezer member in 1992, chose to leave the band in 1998, with the reason at the time being "creative differences." In subsequent years, however, Sharp has made it clear there was more going on behind the scenes than he initially let on.
"I don't really know how to speak on this because I don't know what should be kept private and what should be shared," he told Rolling Stone in 2016. "I certainly have my view of it, as I'm sure everybody else has their sort of foggy things. When you have a group that doesn't communicate, you're going to have a whole lot of different stories."
The mildly tense split between Sharp and the rest of the band snowballed in 2002, when the latter filed a lawsuit claiming he was owed royalties for the single "Undone - The Sweater Song." He said that he was owed 25% interest in the first nine tracks on the band's second album, as well, claiming frontman Rivers Cuomo claimed sole authorship despite contributions for every member.
Sharp almost rejoined the band in 2010

The bassist also charged Cuomo, rhythm guitarist Brian Bell and drummer Patrick Wilson with breach of fiduciary duty and legal malpractice. The lawsuit was eventually settled out of court, though details have yet to be released or discussed by those involved. Interestingly enough, Matt Sharp and Cuomo reunited only two years later during one of the latter's solo concerts. They played four songs together and teased the possibility of a collaboration album. It never happened.
A second reconciliation was teased in 2010, when Cuomo told Spin that he'd considered bringing Sharp back so that Weezer could tour their first two albums in their entirety. Once again, plans fell through. "I think we left that idea behind pretty early on. I don't remember," he explained. "I wasn't involved in the discussion so I don't know what the issues were."
Matt Sharp has never officially rejoined Weezer, and continues to record with his other notable band, the Rentals.