I think a lot of people would be worried about how to treat someone with depression. If they're empathetic people, they might be worried over somehow making things worse... and then unintentionally make it worse.

Other people just don't know how to approach others who are experiencing something they don't/can't understand.

Sometimes people just give others the space that they think they want - and often, when we're depressed people do isolate themselves.

If they just see someone who is depressed and don't KNOW that it's depression, they might just think that the person is very negative or a loner.. and might leave them alone due to a belief that the person prefers it that way or simply because they don't enjoy the negativity.