These short poems are easily remembered, so they come to mind at the right times. When we possess them they live with us.
Using rhyme and rhythm and all the tricks that make words stick, a poem pins down an elusive sliver of experience and shows us its true importance. Or it makes a wise point with wit, like this quirky lesson in cheerfulness from Adrian Mitchell:
When I am sad and weary
When I think all hope is gone
When I walk along High Holborn
I think of you with nothing on.