My brother doesn't really trust anyone until they have had some sort of crushing failure in their life. Something like a divorce, getting kicked out of school, going bankrupt, being permanently injured and missing the Olympics team, or anything where years or decades of effort towards a goal have to be substantially written off.

The idea is that the person you are after a failure like that is much more likely to be stable in the face of additional adversity if for no other reason than you've got a pretty extreme data point that extends the range in which you can emotionally interpolate rather than extrapolate.

The Fins have a concept named "sisu" to describe something like the attitude of someone who perseveres in the face of long odds, high stress, and repeated failure. If I understand correctly, they think of it as part of their national character. In English, the personality trait of grit might be a useful entry point into related academic literature.