A hospital waiting room flattens everybody.
The consultant surgeon and the man who resurfaces driveways sit in the same plastic chairs, holding the same paper cup, waiting on the same news about somebody they love.
Compassion tends to start there — not in principle, but in the recognition that the distance between your situation and someone else's is mostly luck and timing.
People who have been through a specific thing are noticeably better at this with strangers going through it. The rest of us have to get there by imagination, which is slower and less reliable, and is still the only route available most of the time.