Tony sees a little boy crying and he feels sad. At the biology level, what is the BEST explanation of what is happening?

Hormonal Changes ...

Explanation:

The best biological explanation is that when he sees the boy, his visual receptor triggers a response that decreases the levels of serotonin in body. As a result, he feels sadness because his levels of serotonin are low. This hormone works together with other crucial hormones like acetyl-choline and dopamine etc to produce proper emotions related to event happening ...