Othello is a passionate man, valiant, adventurous and loving and Desdemona chose him over all other suitors. Othello and Desdemona are so in love and cannot bare to be apart until Iago's scheming destroys everything to the point of Othello calling Desdemona a "whore."
Cassio disappoints Othello when he is blamed for instigating the brawl . Reputation and honor are everything and Othello cannot understand why Cassio would "unlace your reputation" (II.iii.) and participate in such a basic act, destroying "your rich opinion for the name Of a night-brawler" (II.iii.186-187). Othello does, at this point, still "love" Cassio but immediately strips him of is position - based on "honest" Iago's recounting of the event.
Iago dismisses Cassio's fears at having lost his good name pretending that Cassio shouldn't be concerned but then goes on to tell Othello, in his continued attempt to manipulate him that reputation is everything; thus casting doubt on Cassio's standing.
Good name ... dear my lord,
Is the immediate jewel of their souls.(III.iii.155-156)
and there is much embarassment and disgrace rendering "me poor indeed" if reputation is tarnished.
Having trusted Cassio, this is the begining of the end for the relationship he has with Othello. Othello is well-aware of the friendship betwen Cassio and Desdemona and has never had any reason to suspect anything nor has he even contemplated it. He does stress to Iago that he would need more than words to prove it. "Give me ocular proof" (III.iii.360).
It does not take long for Iago to make Othello so incensed to the point of falling into a trance-like state. Iago sees the effects and is delighted: “Work on My medicine work!”
All the circumstantial "evidence" and coincidence that Iago contrives builds up to the "proof" that Othello needs. Othello is unreasonable and can no longer rationalize - even striking Desdemona - “Is this the noble Moor ..?"
Iago's manipulations have changed Othello, his jealousy becoming all consuming. Almost to the end, Othello still believes that Iago is “brave … honest … and just.” Othello does stop himself - fleetingly- and almost decides not to kill Desdemona but after more accusations he kills her. Still he does not question Iago, even on discovering that Cassio is not dead and clearly somethng is wrong "And sweet revenge grows harsh.”
Othello has been truly destroyed by Iago and is a man who started out so gallantly with many military triumphs and ends with such despair and dishonor. He is“one that loved not wisely, but too well.”