The behavior of a depressed person might be interpreted as melancholy, though that is not what they are really feeling. Irritability is a very common symptom of depression that is frequently overlooked. Depression is a health problem you cannot see or measure, which makes it hard to combat.
The constant work to keep up the necessary aspects of life while addressing depression drains the person and leaves little room for understanding and patience. If someone you know is quick to annoy and has a short temper; that is a side effect of depression.
Feeling sad all the time is the main misconception about depression because it is mostly not feeling anything; most depressed people report feeling numb, and the kind of emotions they experience is sadness and irritation.
As a result, it will be difficult for them to respond to gestures or words of affection. They just do not think about it any more, and they may even get annoyed over it because it is too difficult for their brains to process and respond to these loving gestures.
Depression also makes it difficult to feel fulfilled or enjoy anything; that is why previously loved activities can lose their appeal. It could be a symptom of depression, if you cannot explain their decreasing interest.
A depressed person cannot function the same way as a mentally healthy person; there will be things they will not be able to do as much or at all.
Shaming or pestering them about it will not help, but only hurt. If they are keeping their depression private, it will be much harder to deal with others blaming them for not performing at the level expected of mentally healthy people. Therefore, it is best to be understanding with those people; after all they are struggling with a real health problem, not just slacking off.