Iago says in Act I, Scene 1 that he hates Othello because Othello has passed him over as a lieutenant. Instead, Othello has chosen Michael Cassio, who, Iago says, has no knowledge of how to be a soldier. Iago describes Cassio as someone "That never set a squadron in the field, Nor the division of a battle knows/ More than a spinster" (I.i.21-23). In other words, Iago believes that Cassio knows less about fighting than a spinster, or old unmarried woman, does.
In addition, Iago suspects that his wife, Emilia, has cheated on him with Othello. Iago says:
"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" (I.3.429-433).
The suspicion that Iago harbors towards Othello is ungrounded, but the very fact that Iago is so jealous of "the Moor," or Othello, poisons him towards Othello. He suspects that the Moor has been in his bed, and, without conclusive proof, Iago decides to act as if his suspicions are true. In general, Iago detests and distrusts all women, and he lets this distrust color his relationship with his wife and with Othello.