It is Iago's pernicious and consistent manipulation in his desire for revenge that exposes Othello's insecurity. At first, the general's remarks come across as humility but, as the situation progresses, his lack of confidence is exposed. When Iago plants the seed of doubt about Desdemona's supposed infidelity early in act 3, scene 3, the general responds by saying,
Nor from mine own weak merits will I draw
The smallest fear or doubt of her revolt;
For she had eyes, and chose me.
Othello humbly speaks about his "weak merits" which suggests that he is not arrogant about his abilities. His insecurity is, however, illustrated later in the act when he speaks about himself. This happens after Iago's further talk of jealousy and other matters related to Desdemona's seemingly inappropriate relationship with Michael Cassio. Othello states in a monologue
Haply, for I am black
And have not those soft parts of conversation
That chamberers have, or for I am declined
Into the vale of years.
It becomes apparent...
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