Othello constantly feels the need to justify and prove himself to the people of Venice, his adopted home country.
The tone of Othello's speech is sincere and his love for Desdemona is apparent during his speech defending his marriage. He talks so passionately about his conquests and
of the Cannibals that each other eat, / The Anthropophagi, and men whose heads/ Do grow...
enthralling his audience. He is most likely many years older than Desdemona, is a foreigner and admittedly not as eloquent as other suitors, and yet Desdemona still falls for him.
Brabantio tries desperately to convince the Duke and senators that Othello used spells and medicines to ensnare Desdemona, but Othello’s “round unvarnished tale” of how he sincerely won Desdemona’s love and affection is enough to convince even her father of Othello's good intentions:
I think this tale would win my daughter too.
Desdemona herself is convincing in her argument when confronted by her father as to whom she "owes obedience," and Brabantio cannot pursue further argument regarding Othello's purported witchcraft.