Suffering

Emotional suffering when young is a vital ingredient for becoming wise. It is necessary to learn how painful life can be, and that being alive and experiencing is not intrinsically enjoyable.

This is not sadomasochism.

A deep, lasting, strong experience of emotional pain will force a person to become indifferent to life. If they can bear up under that pain, without sinking into self-pity or suicidalism, then they are able to reflect on what it is that makes life different to not-living.

They become dwellers in the land of thought, imagination, and reflection.

If they can live in between life and death, with no strong attachment to either, neither wishing to "make merry and forget their woes" or to "celebrate destruction and nihilism", then they become thinkers and are likely to have some wise insights.