Excellent wretch! Perdition catch my soul
But I do love thee! and when I love thee not,
Chaos is come again.
My noble lord—
What dost thou say, Iago?
Iago:Othello Act 3, scene 3, 90–95
Did Michael Cassio, when you woo'd my lady,
Know of your love?
By "Excellent wretch" Othello means his new wife, Desdemona, who has just left the scene. Though "wretch" is elsewhere in Shakespeare a term of endearment, it is nonetheless ominous, just like Othello's other oaths in his speech. "Perdition catch my soul but I do love thee" isn't exactly a comforting sentiment; roughly translated, Othello says, "I'll be damned if I don't love you." Thoughts of perdition and chaos aren't the normal signs of a healthy relationship. Othello can only imagine falling out of love with Desdemona as the collapse of all form and order. In effect, his wife is shouldered with the responsibility for Othello's sanity.
So when in this same scene, through Iago's machinations, Othello does fall out of love with Desdemona, he cries "Othello's occupation's gone!" and embarks on a desperate campaign to prove his wife's treachery, a campaign which culminates in his murdering her. Iago hasn't missed a beat; immediately after Othello pronounces his theory of chaos, Iago begins working on the impassioned Moor. He starts by raising doubts about the kind of relationship Othello's friend Cassio has with Desdemona, doubts which explode, within a few hundred lines, into Othello's certainty that he's been cuckolded.