Here's why:
* Fixed character pairings: Sonic Generations features two distinct perspectives: Modern Sonic and Classic Sonic. Each level is designed specifically for one of these characters, with their unique abilities and gameplay styles. You can't switch between them mid-level.
* Story progression: The game's story revolves around both Sonic's journeys, and you experience each perspective individually.
However, you can play as different characters within each perspective:
* Modern Sonic: You can play as Modern Sonic, Tails, and Knuckles in the "Modern" stages.
* Classic Sonic: You can play as Classic Sonic, Classic Tails, and Classic Knuckles in the "Classic" stages.
To switch between these characters within their respective perspectives, you need to:
1. Complete a stage.
2. Select the desired character at the level select screen.
You can also choose between different characters at the beginning of the game, determining which Sonic (Modern or Classic) you'll play first.
Keep in mind that each character has different abilities and playstyles, so choosing the right one for each level can be important!