The reason as to why Othello is so quick to react so strongly to Iago's manipulation is because he feels so betrayed and insecure. I believe that it is a combination of the problem lying within Othello and Iago's skillful scheming that leads to Othello reacting so quickly to the news he is receiving from Iago. Othello is almost a little bit doubtful to begin with in his marriage to Desdemona. Being a completely different race to his wife, as shown through the naming throughout the first three acts, being refered to as 'an old black ram' and 'Barbary horse' (1.1.89-94) have an impact on Othello and how he see's himself. Because the doubt is already there, it does not take long for seeds of doubt to be placed in his mind by the so called 'honest Iago' (1.3.290) that his wife Desdemona is having an affair with Cassio. Iago uses his reputation and skillful trickery to convince Othello into believing that his wife Desdemona is having an affair with a Cassio, a man who is both charming and charismatic and works very closely with Othello as his Lieutenant. He is also a man that people would assume would be a more appropriate match for Desdemona than Othello. Because Othello see's this as a possibility, it does not take Othello long to be convinced that his wife is unfaithful. The reason as to why Othello reacts so quickly and so strongly to Iagos manipulation is not only due to Iago's ability to trick Othello, but mostly due to the insecurities that Othello places on himself, making the problem lie mostly within himself.