ELI5:Why can negative emotions such as depression linger for years or a lifetimes but positive emotions (such as the honeymoon phase) seem to fade?

From a radical behavioural perspective, emotions can be seen as a response to a certain stimulus. We feel happy (we do things that are consistent with the state of happiness) because of a positive event (we go to a theme park, and the rides make us happy), as the question stated. When the stimulus (theme park) and reinforcer (rides) are no longer experienced, the happy response will extinguish, and we no longer feel (outwardly) happy. But, those happy emotions do come back when we are remembering that positive event (remembering the trip brings up good emotions).

However, there is also often a cause for (acute) depression too. Just because we aren't able to pinpoint it exactly, doesn't mean that there is no cause. It might be an internal or external trigger but a simple behavioural explanation for why depression doesn't go away easily is simply because the underlying issue has not disappeared, or taking longer to extinguish because the issues have gone on for much longer than the positive event and is thus more ingrained. Consciously or unconsciously, those things that make you depressed are still there. And that's why the depressive symptoms do not extinguish as easily as acute happiness following a positive event. There are also other models of clinical depression, such that depression is not seen as opposite to happiness, but on an altogether different spectrum.

This explanation sounds very inhumane and robotic but I personally feel that the popular answers in this thread about human urges to keep danger in mind or that we are evolved to do this just aren't good enough explanations, because they make wildly unsubstantiated and sweeping assumptions about people that are almost guaranteed to not be verifiable. There are many interpretations of depression in the psychology literature, and this one is just one of them.

/r/explainlikeimfive Thread