I think every human has been depressed at one time or another in their lives. If you are talking about chemical imbalances, that's a life long condition that needs medical intervention and monitoring. One can expect ups and downs.
I think people are afraid because there are people who are emotional vampires and they will drain you of energy for as long as you allow it. Their problems are magnified in their minds and they give no thought as to the problems other people may be dealing with or even ask other than an initial insincere, how are you, before launching into their own repetitive tale of woe.
I really don't know how long one can be subjected to their constant negativity without being adversely affected themselves. I was a shoulder to lean and an empathetic listener for over 3 years and finally had to distance myself. The person also had a shrink and a call list of people who were also attempting to be good listeners. I felt guilty but found I wasn't the only who had reached their limit.
If people aren't willing to do their own work to improve their life, it becomes a circular conversation.