system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does my heart feel heavy when I'm sad or have a major case of "the feels"?
gpt
Basically, we're not really sure, but scientists think that the anterior cingulate cortex of the brain (the area that regulates blood pressure and heart rate, as well as affects reward anticipation, decision-making, empathy and impulse control) overstimulates the vagus nerve. 

This causes pain, nausea and muscle tightness in the chest -- hence the feeling that your heart is heavy, or those sharp pains when you see your crush with someone else.