The initial purpose of evolutionary psych was to find the reasons humans would evolve mental illnesses with the intent of creating better treatment. For example, Bipolar Disorder may have evolved in areas where humans had limited time outside each season. So bipolar people would have only a few months to do a year's worth of work stockpiling food and firewood. Then they'd be cooped up somewhere safe for months while it snowed and dropped below freezing.

It would make sense, then, if they had bursts of mania followed by bouts of depression. Manic episodes to DO ALL OF THE THINGS NON-STOP CAN'T STOP WON'T STOP and then depressive episodes to increase sleep time and suppress their appetite. Fast forward tens of thousands of years where humans can stay indoors or outdoors all year round. Bipolar conditions that used to be evolutionary necessities are more like vestigial organs, thus causing people distress.

Of course, evolutionary psychology was immediately hijacked by racist eugenicists to do phrenology.