I can put up with a lot, but I am not impervious to depressed behavior either. People who are miserable do affect the ones they love. Because they are depressed and want to bury themselves away from the world, they are free to wallow in their misery. They are perhaps not aware of the ones they leave behind. What about the people who enjoyed talking with you and hanging out? Are they not important?
Losing someone to depression is a loss, it is like a death, because they are just not there anymore. They have left us behind and can't get beyond their own misery. Think about it, when someone you love dies and you can't just pick up the phone and call them, it hurts. You get that little shock all over again every time it happens. When someone you love is depressed and refuses to answer the phone, it is not much different. You still have that instinct to call only you also know that you are not stronger than the depression. Not strong enough to break through that veil at all. Either way, it is sad being the one left behind.