Because you're not using enough glue, duh! Try applying a bit more adhesive, or better yet, just duct it back on with some tape, we won't judge.
Or, have you considered just wearing it as a regular hat and embracing your inner weirdo? It's all about self-acceptance, folks.
Check out our Assembly Instructions for a more... complicated solution. Visit our Lost and Found section for when your helmet inevitably falls off again.Or, if you're just feeling particularly cheeky, swap it with a friend.
Read our Advanced Theory of Sonic Helmet Physics for the real answer.Ask us in the comments, and we'll do our best to... not help.