What are the roots of compassion?

I've been reading a lot of Issa and Ikkyu, and their poetry is consumed with compassion, a topic that I have been focused on lately. It's Sunday, as good a day as any to give it a go.

I've been wrestling with the source of compassion in zen. What is your understanding of the roots of compassion, /r/zen ?

Why is a person compelled to move and help another, if it's all the same thing?

How is listening to the sound of a child's laughter better than listening to the sound of a child being tortured?

Was Ummon's master moved by compassion when he slammed the door on his leg, causing his first kensho?

Why does zen care at all? Does it?