One major cause of mental suffering and distress: the constant search for something better. The human obsession with optimization runs deep. We’re always on the search for ways to make our lives better, more efficient, more relaxed, more productive, more luxurious, etc. Except that we’re very bad at knowing what will actually make us happier. The truth is, we have an internal drive to want more, but most of us are actually happier with less. This makes us obsessed with maintaining lifestyles that require working long hours and doing tasks that can actually make us less happy.
There’s a real asymmetry in rewards. It can take many hours of hard work and labour to afford things that give us mere fleeting moments of happiness. Think of the amount of time and energy that go into affording a vacation, a week of respite from the hard work and labour you do to afford your vacation. Things often break quicker than we acquire them, so you either work like crazy to tread water, or you give up the chase and choose a more minimal lifestyle.
For more on this, I recommend Dan Gilbert’s book Stumbling on Happiness.