why does othello discharge cassio from his service?

Iago says that he intends to get Cassio drunk, that will hopefully cause a quarrel between Cassio and Roderigo, who has been stirred up against Cassio. Iago wants to see Cassio discredited through this, so that he might take Cassio's place. Montano and others come, and Iago entertains them with small talk and song; soon, Cassio is drunk, and Roderigo has approached. Cassio fights offstage with Roderigo, and comes forth, chasing him; Montano tries to hinder Cassio, but Cassio just ends up injuring him. All the noise wakes Othello, who comes down to figure out what has happened. Montano tells what he knows of it all, and Iago fills in the rest‹making sure to fictionalize his part in it all too. Cassio is stripped of his rank, and all leave Cassio and Iago alone.