When depressed, why do the smallest of things sometimes just send you into this downward spiral that won't stop?

It's like the saying says "The smallest things make a big impact." I believe in that statement, basically due to the fact that as the little things pile up it becomes a huge mess to go through sifting through so many different issues that is not helping instead of one. In addition small ones stem from a moment of triggering and leads one mind to think of other occurrences when they felt this down. Hence thee downward spiral.

they become the last drops. trying to be happy from small things when there is a larger source of sadness in your life is trying to solve the problem with a solution that's not the solution to that problem.

It reminds you of previous bad experiences, and the body is more and more responsive to the slightest stimuli triggering the state for the smallest of reasons.

It just seems like there is one more thing that you cant manage. The mountain keeps getting so big for even the smallest of things. I can never get out from under it. For me, it seems like nothing can go right. Some times I have to remind myself to step back and look at how everything is. Not just the fails but the successes as well. For some reason, we need things that are good to be huge to make sure they can outweigh the bad. Building on the smallest good is a way to change our outlook. Also, ask your self from these little things that happen, how does that really affect or matter to me?

because we can't think straight, our feelings takes charge and we usually think whatever negative thing we feel/think is true (based on one of 7cupsoftea training ;) )

Because we feel at times things are not in control. Depression is an altered state of mind when we are not seeing things clearly. Like a muddy pool of water, we need to wait for the mud to settle to see our reflection with clarity. Sometimes, it is a trigger of unresolved issues from the past therefore reacting without giving any thought. It does not stop because too often we may choose to dwell in the depression as it is the easier option rather than taking action to recognise and stop it. It is the little things that have the bigger impact on our lives.

It´s as if those things remind you of other, greater things. Depressed people are very good at associations. It´s a human thing to dwell on stuff.