Even the word run itself. Solids do not. Maybe another reason why we run. Life is hard. There are stresses at work, and sometimes, we lose our jobs due to no fault of our own.
Relationships are tough.
Even the most solid ones have bumps in the road. Children are a challenge. The may provide our greatest joys, but also cause us our deepest despairs. Life is hard. Something that is hard is not easily molded.