Keep in mind that they can’t “snap out of it.” Remember that the other person has a real illness. Like someone with cancer, they can’t simply “get over it.” Try not to express your frustration or anger in ways you’ll regret, but don’t suppress your own feelings either. You can say for example, “I know that you can’t help feeling down, but I feel frustrated.”
If the person is an unrelenting pessimist, as so many people with depression are, try to point out the positive things that are happening. The negative childhood programming–the “inner saboteur”–will probably prevent them from seeing these for himself. The depressive illness has a vested interest in the lie that nothing will go right.
via Dr. Bob