Simply put: because they are at the end of their tether. This is also referred to as family burnout. There are various facets to this:

• The depressed person has no insight into their illness.
• The illness is concealed, often even in the closest (family) environment.
• The “function mode” in the relationship is out of order, i.e. tasks that two relationship partners used to take on now have to be handled by one person alone.
• In addition to the physical exhaustion of the relatives, there is also emotional exhaustion (e.g. coping with relapses, a perceived standstill in coping with the illness, a lack of confidence in a shared future). 

Sometimes depression is a kind of magnifying glass on the points of conflict in the relationship.

A separation is inevitable if the health of the healthy person is also at risk. A separation is also likely if the person suffering from depression refuses any professional help such as therapy, a stay in hospital or taking medication.

• The depressed person is afraid of putting too much strain on their healthy partner or even ruining their health.
• The depressed person sees the relationship as the reason for the depression.
• The person suffering from depression has a distorted perception of the relationship, which is, however, a symptom of the illness. 
• The depressed person no longer feels any emotions or connection to their partner.

• Don't take the depressed person's behaviour personally. Depression initially says nothing about their love. Get professional support to “sort out” the situation. 
• Depression often comes on gradually. If a partner is noticeably irritable, talk about this change. 
• Accept that the depressed person is unable to contribute to family life as usual due to their illness. They simply lack the energy, motivation and zest for life. 
• However, don't take all the tasks away from the person who is depressed. This may make them feel even more helpless, which in turn will affect their already fragile self-esteem. 
• The treatment of depression belongs in professional hands. As a relative, you are not the therapist. 
• If your partner has no insight into their illness, encourage them to seek support. Depression is a mental illness, not a weakness! If your partner expresses suicidal thoughts, go to the nearest psychiatric emergency room! 
• Even if the depression is well treated, there are worries on both sides. Share them! Discuss your fears, e.g. of a new episode. And that you are worried that the healthy partner will leave the relationship. The healthy person can also say that they sometimes feel overwhelmed and discouraged. 
• The healthy person should not give up on their interests. Depression also costs them a lot of energy and they should do something for their mental health without feeling guilty.

• Communicate clearly that you want the separation.
• Tell your partner that you accept that he or she does not want to be helped, but that you can no longer walk the path together with him or her.
• Involve the people around you as support for the first time after the separation (if children are involved, check who can help with childcare, transportation, etc.).
• Stand by your decision to separate and stay away from people who do not want to accept your decision.
• Stop taking care of your partner's depression. Otherwise you will only be changing the household, not the role.