Why does Fabian agree to help trick Malvolio?

Fabian agrees to help trick Malvolio because he finds amusement in the idea of exposing Malvolio's self-righteousness and pretentiousness. He enjoys the camaraderie with other characters, particularly Maria and Sir Toby, and sees the plot as a way to entertain themselves at Malvolio's expense. Additionally, Fabian's involvement allows him to take part in the playful subversion of social norms and to challenge Malvolio's authority within their social circle.