Your friend wished he had focused on his career while the people who focused on their careers wished to have lived more experiences.
Married people wished they were single to enjoy the freedom and live a simpler life. While single people wished they met a SO to be less lonely.
People with stable high paying jobs like many developers, daydream about financial independence and quitting their jobs. While most other people dream of just finding a stable high paying job.
People from 1st world countries complain a lot about their own countries while most people in the rest of the world would literally die crossing the border to get there.
Young people wish they were rich. Rich people wish they were young.
And the list goes on and on. This seems to be something very human... Why do we think so much of the things we lack/didn't do but seem to forget what we have/have done?