Iago's motivation is essential to the play but it is not rational. Critics have been discussing this question for centuries. Shakespeare himself must have realized that Iago's motivation was a weak element in his play. He tries to justify it in several ways. Iago says he is angry at Othello for appointing Cassio his second-in-command instead of himself. He also says he suspects Othello of having had an adulterous relationship with his wife Emilia (as if Othello would do such a thing, and as if Iago would really care!). Even at the very end of the play Shakespeare is still wrestling with the problem of Iago's motivation. Othello asks Cassio:

Will you, I pray, demand that demidevil
Why he hath thus ensnared my soul and body?

IAGO:

Demand me nothing. What you know, you know.
From this time forth I never will speak word.

Even when threatened with horrible torture, Iago refuses to explain his motive for plotting Othello's destruction. Lodovico says:

                     For this slave,
If there be any cunning cruelty
That can torment him much and hold him long,
It shall be his.

No doubt Iago refuses to explain his motive or motives even when they get him back to Venice and torture him to death. The poor man can't explain his motivation because his creator Shakespeare didn't understand it himself. Iago is cutting his own throat by undermining Othello. His position is entirely dependent upon Othello's position as governor of Cyprus. But many critics have tried to justify his self-destructive behavior because they can't believe that the great William Shakespeare could have made such a mistake in constructing a play. Shakespeare made many mistakes.

In Othello by William Shakespeare, Iago is the villain who schemes against the Moor, Othello. In an early speech in the play, Iago reveals the motive for his action, stating

I have told thee often, and I retell thee again and again, I hate the Moor. My cause is hearted; thine hath no less reason. Let us be conjunctive in our revenge against him ...

Iago is quite rational, and even cunning, in pursuit of the downfall of the Moor. On the other hand, the initial hatred is not entirely rational. Some of the hatred is caused by jealousy, especially in light of Othello's promotion of Cassio, and some may be racist as well, but hatred carried to the extreme demonstrated by Iago is outside what we might practical disadvantage.