Every minus sign inverts the direction of the change. Every plus sign leaves the direction of the change as it is. More doing things leads (+) to more feeling good, which leads (-) to less depression, which leads (-) to less de-motivation. But more de-motivation leads (+) to more depression, which leads (-) to less feeling good, which leads (-) to more depression.

A fair number of systems created here haven't grokked the behavior of (-), leading to models that don't actually behave as intended. I admit, it took me a few tries to notice, myself. The description given for relationships doesn't help, suggesting that (-) does nothing more than changing 🡩 to 🡫. Having three options, more (🡩), less (🡫), and flip (↕) would have been clearer and more versatile.

Doing things has a positive (+) impact on feeling good, which has a negative (-) impact on depression. The sign there is important.

Doing things only has a positive impact if the outcome is at least positive.

I can't find a good solution for the problem and that's a bit depressing... This feels a bit more accurate to me though: http://bit.ly/2nsIOXc