Only semi-rhetorical counter question: How can the depressed person possibly know what the other person can or can not imagine? By what right do you tell me what the borders of my perception, memory and imagination are?
If I can not enter your head, neither can you enter mine. If I can't imagine what it is like to be depressed, can you imagine what it feels like not to be (pplchknz just mentioned this) and see the world with the eyes of a healthy person?
What I mean by that is that both party's vision is partially impaired. Of course you can't just cheer up or snap out of depression, that is patently ridiculous. But once you accept the fact that you do have a desease there are ways to get better. It is hard work and it takes a lot of time, but you are not some damn emo snowflake nobody could ever possibly understand because your problems are so unique. They are not, and that is a good thing, because it is the reason that there are solutions out there.