Sequels in gaming are different to sequels in the movie industry. A gaming sequel allows a developer to tweak and enhance the design that they implemented in the first game, making for a far better experience. A huge chunk of the best games ever made are sequels, because they're games that developers perfected, using the experience earned from previous titles in the franchise.
Without sequels we would never have Mass Effect 2, Red Dead: Redemption, Fallout 3, Persona 4, Ocarina of Time, Elder Scrolls III and IV, Final Fantasy VI, VII and IX, Half-Life 2, Metal Gear Solid 3, GTA: Vice City and San Andreas, Civ II, Pokemon Silver, Call of Duty 4, Halo 2, Silent Hill 2, Super Mario 64, Resident Evil 2 and 4, Assassin's Creed 2, Uncharted 2, Dead Rising 2, Duke Nukem 3D, Gears of War 2, Freespace 2, System Shock 2, Killzone 2, Devil May Cry 3, Starcraft 2, Left 4 Dead 2, Battlefield 2, BF:BC2, Baldur's Gate II, God of War 2 - these are all games that significantly improved on their predecessor(s), with some of them being considered outright classics in their own right. Sequels are good. They allow developers to create the game they intended to in the first place, and providing better experiences for all gamers.