what reason does iago give for wanting to get revenge againgst Othello?

THere is much speculation about this. Some critics think Othello had slept with Iago's wife. Iago seems upset that he has been passed for promotion. In any case, Iago represents evil and wishes to see the Moor undone,

I hate the Moor;

And it is thought abroad that 'twixt my sheets

He has done my office: I know not if't be true;

But I, for mere suspicion in that kind,

Will do as if for surety.