It's hard to watch the familiar, loving person you know become a distant stranger, one who withdraws or lashes out, or both. It means that your own need for emotional intimacy, companionship and social activity will be largely unmet by your partner for the duration of the depressed cycle. It's natural then, to focus your problem-solving attention on the partner, while you lose sight of maintaining your own health and simultaneously, and inadvertently, changing the relationship to something less than healthy and happy.
You, as the partner of a depressed person, need special care and attention. As you care for the depressed person, you must make sure you take care of your own emotional and social needs and of the relationship you value.