There was once a group of Japanese soldiers from WW2, that hid for decades in the jungles of the Philippines because they refused to believe the war over. The story of the man who led these troops spread throughout japan as myth, many saw him as an honorable hero who would fight for his country till the very end, while others believed that some solider chose to live in the jungle for decades because every time someone left a message saying that the war was over he'd be like, "Nah, this is an enemy trick, 20 years later and they still are using this same old trick."
Eventually a Japanese deadbeat, dropout decided that roaming the jungles of the Philippines looking for this solider that hasn't been found in decades would be pretty fun, (this is all after he already left japan traveled the world as a hobo, and came back). It's not like he had a responsibilities anyways. After reaching the island that the soldiers were stationed back in WW2, the japanese traveling hobo just walked around and shouted his name, and found him in less than a week (it's safe to assume no one was really looking for him). After the two met, they talked and the solider was convinced to head back to japan, while the other guy goes to find a panda, rhen the abominable snowman. The solider comes back to japan as a celebrity, but is horrified to see that the traditional japan he fought for long ago is gone, and while he recieved attention nationwide nobody really cared about his movement to "make Japan great again." If you want to know what happened to the other guy, he found the panada, but died in the himalayas searching for the snowy sasquatch.
The point Mark Manson wants to make about these stories is that goals will never be enough for a person. The solider wasted most of his life working for purpose only to realise he fell flat on his face a long ass time ago. And while the other guy met some of his goals, he wanted more and that eventually sent him to his death. The joy we receive from actually reaching a dream is short lived, just ask most people who become incredible rich, they always say money and getting things doesn't bring happiness. The people who really get excited are those in the process of reaching a dream. When you aspire to become an professional singer or something, the most emotional part isn't the lifestyle of living on the road, having millions of fans and dollars, etc... because having everything gets boring, what you really remember is the struggles of getting your name out their, perfecting your voice and songs, your auditions that dictate whether you'll reach that dream of yours. A the true purpose of a goal isn't to overcome it, but to occupy our time with a purpose.
The soldiers who stayed in the jungle, were willing to stay there for years without end, because they had purpose there, they were a part of a cause. And as for the other guy, he always occupied himself with something, and he never seemed dissappointes with life.