Depression is often triggered by loss, not only of relationships but also of other things which are important to us - our role in life, our health or ourself-esteem - and the fact of being depressed can then result in further losses, because we become difficult to live with and unable to play our part in life. - P90
Grief is normal, and usually resolves over time, but if it doesn‘t then it can become indistinguishable from depression. - P92