To answer such a question it is important for me to first explain when a disease is contagious or not and then come to answering your question directly.
So in modern day medicine,diseases or rather medical conditions can be classified into
--Non communicable diseases/conditions.
So what actually determines whether or not a disease can spread or not?
It is quite simple ,there are three factors that determine every disease/medical condition these are
For a disease/ condition to occur there is a co-relation between these three factors.In the broad category of communicable diseases the agent is usually a microorganism which thrives in a particular environment and infects the suscpetible host.This organism can spread from one host to another and hence a huge propotion of communicable diseases as the name suggests are contagious.
Depression on the other hand is a mental illness that involves the body, mood and thoughts and that affects the way a person eats, sleeps, feels about himself or herself. Research has proven that it is caused due to some biochemical imbalances in the brain.The cause of this could be situational responses to stress, trauma, loss and other life events. Since there is no infective agent, there is no physical spread of the condition from one person to another and hence we can safely say depression is a non communicable disease/ condition. All non communicable diseases are not contagious and hence depression is non contagious. But this is when we purely look at it from a medical point of view.
To cite a point in favour of this, depression can often be seen in relatives or people living in the same house or near vicinity. This must not mislead us to think that it is contagious rather research has proven that there is a strong genetic predisposition to this condition and hence often seen in family members or close knit societies.
It is quite often that depression has been referred as the ‘common cold’ of mental illnesses and 40% of adults will experience loneliness in their lifetimes. Consequently, the likelihood of you being in close contact with a person who is either depressed, lonely, or both, is rather high. A recent study assessed incoming college students’ outlook and thinking style before they moved in with their randomly assigned roommates and repeated the assessments three months into the semester, and another six months later. They found that students who did not have a negative thinking style but roomed with a person who did, often ‘caught’ their roommate's negative outlook and had twice as many symptoms of depression at the six month mark. The results were so alarming and so significant (given the short period of time), the researchers hypothesized this effect might not be limited to situations of major life transitions.In other words, when you spend a significant amount of time with someone whose outlook is negative and pessimistic (as is the case when a person is depressed), their maladaptive perceptions and thinking can influence your own such that over time, you too become more vulnerable to depression. Now this is looking at it purely from a psychologist's point of view.
Hence to answer your question as a medical personnel, i would say that the answer to this question is extremely subjective. And like majority of medical personnels i would say that depression is not contagious.