In terms of a relationship, I admit that this tends to be problematic. Sometimes I have trouble communicating with my partner and I get really upset for no apparent reason sometimes. It's important, in a relationship, to not take the symptoms of depression to heart because 9 times out of 10, your partner does not feel that way about you. My partner has had problems finding ways to help me out and often believes that his existence should be enough to cure my depression. While I'm less depressed around him, I can't necessarily be around him all the time to help my moods. The only person who is around me 24/7 is me, so the healing process has to start with me.
My counselors have told me that I have ingrained a way of thinking so deep into my existence that it has imprinted my id to make me think that nothing matters in the end. That my existence means nothing and I should accept that, and kill myself, so not to be a burden on other people because I am a piece of human garbage. Depending on your partner's way of thinking, I encourage you to be positive around your partner. This may help them change their ways of thinking to start a healing process.
You have to give the person space when they are feeling down. If they are having a "bad day" (as I call it), it is best to give them space to work out what they are feeling. Don't get me wrong, it is still best to be supportive of them and speak to them when they are speaking to you. Just take into account that if they aren't talking to you because of their bad day, it is not because of you or anything that you did.
Personally, I self-harm to escape my lack of feeling anything because it makes me feel like a human, when I feel pain. It makes me feel normal. If your partner has a problem with such things, you can encourage healthy coping mechanisms. "Let's take a walk.", "Let's have lunch." if you are around them. If not, you could encourage them to write in a journal instead or have them call a friend. If these things fail, do not get upset at them for relapsing into old habits because it's not something they can necessarily control.
If you want more information on the subject, I've included a few links to different websites that I have found helpful:
On MDD: http://mentalhealth.com/home/dx/majordepressive.html
On Coping Mechanisms: http://www.everydayhealth.com/depression-photos/ways-to-cope-with-depression.aspx
Helping your Partner: https://www.adaa.org/finding-help/helping-others/spouse-or-partner
And always, the National Suicide Hotline:1-800-273-8255
All statements expressed here are things that I have collected over the years and from my own experience. They are not equal to every situation because depression varies from person to person.