Why are some people more empathetic than others?

In the same way we know people with extremist personalities, we all know people at both ends of the empathy spectrum: The ones who will rush right over with a freshly baked lasagna when your cat dies, and the ones who make you work until 9 p.m. on the night your kid has the solo in the seventh-grade choral concert. Typically, we'd call the latter scrooge "cold-hearted." But the biology's all wrong. Ain't the heart that's cold; it's the brain that is.
As it is with virtually every one of our biological functions, there's a survival value to feeling empathy for others; teaming up with a community to get a job done or fend off an attack is more advantageous than going it alone. It's more than just adaptation at play, though. It's brain function. Our morality is largely dependent on how connected we feel with others; the more connected we feel, the higher our degree of generosity and compassion.