Why does Othello marry Desdemona?
More than almost anything, Othello is an outsider. He will never be fully accepted into Venetian society as he so craves because of his race. Having Desdemona fall in love with him, having her chose him over all her other suitors, and being exonerated that there was no force involved with the marriage would seem to give him a better place in Venetian society, which would make people in high places look past his race into what a competent leader he is.
But, that's a double edged sword. Being acutely aware of his status as an outsider, it makes him an easy target for Iago. Othello can't stop being afraid that everyone else in Venice is still thinking, why would one of Venice's most eligible bachelorettes marry a Moor?