What is the one thing (in addition to the lieutenancy)which cassio states again and again that he is upset over losing ?

I think that Cassio alwasy fancied Desdemona. He certainly talks about her in terms of her beauty and virgin purity.

O behold...You men of Cyprus, let her have your knees,
Hail to thee lady! And the grace of heaven,
Before, behind thee, and on every hand,
Enwheel thee round. (Act 2))