When someone is depressed, that depression is connected to everything about the person. It can affect what they do, where they go, how they feel, who they interact with, and many other factors. Although depression does not and should not define a person, it will be tied to the person who is suffering from it. Anyone else who is around that person will also feel a connection to the depression, but it might be felt in a variety of different ways. The main point here is that depression has connections, whether someone wants them or not.
But there is good news. When you really start to think about it, everything is connected. This could involve a deeper philosophical conversation, but to keep it somewhat simple it is sufficient enough to say we are all connected as humans and what goes on in the world. So, from that, if depression can be connected, then happy thoughts and actions are also connected. If someone is depressed, seeing or hearing about a happy event can give that person connections to happier thoughts.
Of course, someone who is depressed may end up feeling even sadder that they could not experience the happiness for themselves or that they simply cannot feel much at all. That is understandable. But, the very act of witnessing or being exposed to happy thoughts creates a connection that can be so important. It may not appear to help much in the moment, but the more happy and meaningful connections that can be made for a depressed person, the more a benefit is possible.
I understand this may all sound pretty vague, but overall the idea is that depressed people should be as connected to as much happiness and meaning as possible. It may not be the absolute ticket to recovery, but it can help expose the depressed person to a more helpful way of experiencing life.