Maybe it's something to do with the fact that generally speaking, sentient organisms have a drive towards progress/higher value.
(Bear in mind, this could simply rest at securing enough for ongoing survival - so not necessarily rampant high-scale progress).
We seek/head towards value. And by consequence, move away from where we currently are. Meaning that where we are must be in some way, 'not good enough' - and also that therefore, life is then really about escaping from a permanent state of suffering.
As humans, we have great capacity to reflect...but also to ruminate on what's not perfect/missed value/future consequences of such 'failures'. Hence, ongoing misery.
If you're constantly taking action towards 'better' then it may go a long way towards at least distracting, but probably also assuaging this innate tendency to worry about being in stasis and getting left behind. (While the rest if the universe apparently progresses, - making our static state of current suffering even more pronounced.)
On a side note, I suspect this may be at the root of why people do so much DIY refreshing when things aren't in disrepair. Needing something to do with their time to feel like they are 'progressing'.