Try not to view someone else's depression as your own affliction. Rather, be grateful that you don't have clinical depression and try to realize what the other person is going through. They aren't meant that way. Recovery from depression is not just a matter of taking anti-depressant medication and going to therapy . Both the depression and recovery from it can totally change a person's life.
Treatment involves a lot of fundamental changes in a person.
At times, you'll wonder if it's the same person you've known for so long. At times, it may seem that the person is actually pushing you away. This is very likely true. Most depression patients believe that they unduly affect those around them and will do anything to prevent that from happening. Thus, they isolate themselves from others.