Coleridge famously described Iago's ruminations as "the motive-hunting of motiveless malignancy". There could be something in that as it is possible that Shakespeare was embodying the stock figure of Vice from the old Morality Plays. Yet it seems an unlikely fit in a drama that is domestically realistic - note Othello's homely list of his wife's accomplishments and pastimes, when he defends her virtue to Iago much later.
Other reasons given are
being passed over in promotion by Cassio, an "arithmetician" who lacks his own military experience. This is more credible than the others which include:
a spurious suspicion that Othello has slept with his own wife, Emilia;
an even more dubious one that Cassio has also cuckolded him with her;
racial hatred of Othello - but inexplicable why this should suddenly have surfaced
love of mischief for its own sake, which could be backed by the quotation earlier in the play, after the drunken brawl: "Pleasure and action make the hours seem short". (See the reference to the Auden view at the end of this post).
his own feelings for Desdemona - specious again as he rules out "absolute lust"
I take, as Iago's anchor motive, his saying of Cassio:
"He hath a daily beauty in his life
That makes me ugly".
Listen to the falling cadence in those words and they come across as the truth. With Shakespeare, who knew living drama on the stage, we must hear the lilt of the iambic pentameters (and Iago here speaks in blank verse) and respond to it. The next lines about being afraid of a reveal have quite a different tone of rationalisation.
Here I will go out on a limb and suggest that Iago, at a profound and unwilling level, also feels that Othello and Desdemona have that "daily beauty" in their lives. This is barely thinkable within his overt view of a man of colour - but Iago has witnessed the reconciliation with Desdemona after the storm. He has heard and watched the rapturous greetings and the aria-like quality of their speeches, particularly Othello's. He must "set down the pegs that make this music" and create discord instead of harmony, by turning her "virtue into pitch." Meanwhile he resorts to debasing language whilst he plots the destruction of love and beauty.
For further reading and food for thought, I suggest W.H. Auden's essay: "The joker in the pack" in his collection, The Dyer's Hand.