Wasn't "Dumb and Dumberer: When Harry Met Lloyd" a sequel, or does that count as a prequel. Either way, it sucked in comparison, why would/should this one be any better?
It was a prequel.
It was directed and written by people not even attached to the first film, and had an entirely separate cast.
This sequel will be directed by the Farrelly brothers like the original was, and will star both Jim Carrey and Jeff Daniels. That doesn't guarantee that the movie will be good, but it gives it a good chance.