I have been chatting up Duke Brennus, and I tlaked to his adaughter once, and now whenever I ask him if I can, my relationship goes down with him. I don't know if I can fix this or make him say yes or what. If I can, please tell me?
Please post questions of this kind in the Quick Questions/Quick Answers thread. It is designed exactly for such simple requests and to prevent cluttering of the board.
In this case, I'd advise you to increase your relation with the lord before asking him again.
Just do as many of the quests he gives you (you can ask him if he needs help at pretty much any time) as you need to until you get your relation with him to around 25. Be warned, however, if you do get married, the wedding gift price is usually pretty expensive, so save your pennies for it.