In a nutshell, it's because they offer a rare time to truly be yourself, away from the pressures of society to live a 'normal' life... bleurgh!
You wanna dance? DANCE!
You wanna dress up? DRESS UP!
You wanna high-five everyone you see? DO IT!
Festivals have the perfect blend of music, sunshine & ridiculousness, bringing out the BEST version of you! And what could be more fun than that?