system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 (actually 5): Depression

I have chronic depression and sometimes it's unfortunately obvious. 

My 5-year-old niece asked me why I'm always sad and crying when nothing bad has happened to me.

How do I explain this to her?
gpt
Because your brain is sick, so it makes you sad when there is no reason to be sad.  So you try and you try and you try to be happy but it does not work because your brain is sick.  Just like if you have the flu, you can try and try and try to feel better but it won't work, because you're sick.  

So people with depression go and see a special doctor and sometimes take pills if the doctor orders.  It can sometimes takes years and years to get better.

Then you tell her that if she ever feels very very sad, and can't seem to get happy she can come talk to you.  It's important to talk to people when you get super sad because your brain might be sick and playing tricks on you.  She can talk to you about it because you understand and might be able to help her.