It is just like English sayings and phrases, there are Japanese sayings and phrases as well. It is called [kotowaza ことわざ].
鬼の目にも涙 Oni no me nimo naamida
Oni – a deman
Namida – tears
Even the hard-hearted can be moved to tears
Even a cruel demon cries.
Even a person who is not caring usually, he/she sometimes becomes caring person.
My teacher is well-known as a strict teacher at my school. But when my class won a choir competition he cried over the joy. Oni no me nimo namida da.