Validate his/her feelings, let his/her know you agree with him/her when that is true, but avoid criticism when you don't agree. you don't have to pretend to agree if you think he/she's wrong, but you can say it in a diplomatic way, like "I think differently, but I understand your reasons for feeling that way." (and then don't continue to argue about it.)
Reading this is timely for me. I've been struggling today with my not agreeing with my husband's reasons for something and his disappointment in me for that. But I have been thinking about it all day and realized that I have my own reasons for the same shared goal and they are reasons that should make me happy and him too and so I plan to tell him that. It doesn't matter if our reasons are the same as long as the goals are.
