I found the same thing happened sometimes in my relationships when I was depressed.. I think it might be as simple as realizing when your partner is happy, they have more strength and are reliable, therefore you feel like it's okay to let your guard down and feel low, knowing she may be able to lift you up while she is feeling better. Just an idea, it might make no sense. When she is depressed, you might not be realizing you are putting on a strong part of you that requires you to be happy to help her out of her depression. This might not be one of my brighter things to say to help someone. sorry