Because humanity is the most complex and interesting subject that exists, and limiting yourself to a small portion of it is a good way to miss out on all kinds of interesting things.
That would also apply to not having any friends outside of your country, or outside of your generational group, or outside of your socio-economic class, or ...
Also, “all kinds of interesting things” have opportunity costs.
This is a repository for major, life-altering mistakes that you or others have made. Detailed accounts of specific mistakes are welcome, and so are mentions of general classes of mistakes that people often make. If similar repositories already exist (inside or outside of LW), links are greatly appreciated.
The purpose of this repository is to collect information about serious misjudgements and mistakes in order to help people avoid similar mistakes. (I am posting this repository because I'm trying to conduct a premortem on my life and figure out what catastrophic risks may screw me over in the near or far future.)