Why did iago want to cause trouble?

Answer 1

This is a matter of debate among Shakespeare scholars. Iago gives numerous reasons for hating Othello, but ultimately refuses to answer when asked at the end:

Othello asks

Will you, I pray, demand that demi-devil

Why he hath thus ensnared my soul and body?

Iago replies:

Demand me nothing: what you know, you know:

From this time forth I never will speak word.

Answer 2

Othello doesn't know that Iago is an enemy. He thinks he's a friend, and an honest man. This allows Iago to get Othello to be an enemy to himself, which would not happen if Othello knew that Iago was an enemy.

Answer 3

Iago used very subtle psychological manipulation to control the behaviour of everyone around him, usually in order to destroy them. He says, "I hate the Moor", and in his soliloquys says that he has heard that "'twixt my sheets he has done my office" which he is prepared to believe without proof. Later he says, "I do suspect the lusty Moor hath leap'd into my seat", and also that he believes that Cassio has slept with Emilia as well. Apparently the reason that he is able to induce Othello to be unreasonably jealous is that he is unreasonably jealous himself.