What is the reason Iago gives for hating Cassio?

Iago has a very clear understanding in his mind as to what a man should be. And it is fair to say that Cassio comes well short of that ideal in a number of ways. For one, Cassio is of noble blood, and Iago has a bit of a chip on his shoulder concerning his own relatively lowly status. Iago's judgment is brought to a boil by Othello's decision to award the position of lieutenant to Cassio. Iago has lobbied hard for the role, but in truth he never really stood a chance. Othello's snub adds to Iago's jealous rage and sense of injured pride.
What especially rankles with Iago is that Cassio, unlike him, has no experience with the sting of battle:

One Michael Cassio, a Florentine,A fellow almost damn’d in a fair wife;That never set a squadron in the field,Nor the division of a battle knowsMore than a spinster (Act I Scene i).

Iago wonders why should such a man be appointed lieutenant? And why should someone with firsthand experience of soldiering be passed over? Iago feels most aggrieved. He has been snubbed in favor of someone who is little more than a "bookish spinster," someone less than a man, in Iago's opinion.
But Othello's rejection of Iago for the role of lieutenant is important for what it tells us about Iago's character in general. He is fundamentally insecure, not happy in his own skin. So the sight of someone Cassio rising to prominence at his expense—with Cassio's good looks, intellect, and air of aristocratic poise—acts as a provocation that stirs up all of Iago's insufferable pride and self-loathing.