Throughout the course of the play there is reasoning that Iago fits the description of "motiveless malignity," however, are there reasons for why...

Iago states in act 1, scene 1 that he is jealous Othello made Cassio his lieutenant. Iago believes he has had more battle experience and is therefore better qualified for the position than Cassio. He accuses Cassio of getting all his military knowledge from reading—what Iago calls "bookish theoric"—whereas Iago implies he himself is battle-hardened and presumably wiser about military matters:

One Michael Cassio, a Florentine,A fellow almost damned in a fair wife,That...

Iago states in act 1, scene 1 that he is jealous Othello made Cassio his lieutenant. Iago believes he has had more battle experience and is therefore better qualified for the position than Cassio. He accuses Cassio of getting all his military knowledge from reading—what Iago calls "bookish theoric"—whereas Iago implies he himself is battle-hardened and presumably wiser about military matters:

One Michael Cassio, a Florentine,
A fellow almost damned in a fair wife,
That never set a squadron in the field,
Nor the division of a battle knows
More than a spinster—unless the bookish theoric,
... mere prattle, without practice
Is all his soldiership. But he, sir, had the election.

In act 1, scene 3, Iago asserts he has heard rumors that Othello is sleeping with his wife, Emilia. Although he says he does not know whether it is true, his reaction fits into his pattern of expressing twisted views of women, considering them all sexually unfaithful. When he says the Moor, Othello, has done his "offices" "'twixt his sheets," he means he has heard Othello has slept with Emilia. Iago states:

I hate the Moor,
And it is thought abroad, that 'twixt my sheets
'Has done my office.

While I believe Iago does suffer from what Coleridge called motiveless malignancy—that is, he appears to have a hatred of Othello disproportionate to anything Othello has done to him—Iago does, in fact, provide rationales for his actions. He states he resents the unfairness of Cassio being promoted over him and fears that Othello is sleeping with his wife. Whether we believe what he says is the question.