It is a good question. I have a couple theories.
1)
It's easier to sit in the muck than it is to stand up.
If your gonna stand up you have to fight to get yourself un-stuck.
You may even have to ask for help from someone you know or a stranger.
Once you get yourself out of it you still have to wash yourself off.
Then you have to find something clean to put on.
Once all that is done you have to present yourself back into the world.
There is a chance that someone will try to push you back into the muck or you might even fall in by yourself.
Then the negative thinking comes in. You have choices. Should you throw yourself back in it before someone else does?
Should you just try to go on.....but you'll fail...you did before.
Oh hell, why even bother getting up.

2)
Some people need more attention than others due to insecurity, lack of it as a child, there are many causes. When you are ok, or people think you are or even want to believe you are, they don't feel the need to constantly reassure you, they don't need to always be there.
So if you can remain sad, desperate, lonely etc. then you will hear the "poor baby" "poor you" that you need to make yourself feel better. It's just a temporary bandage but it works. When you need a refill all you have to do is cry again and someone will come and take notice.

Just my 2. I think the first is more common.
Heidu

Every path to a new understanding begins in confusion- Mason Cooley