[hackable_sand]

It sounds bad because the root comment is priming a negative perspective of the world.

They could have said:

"Most people are kind. Be kind and most people will be kind back." Or something with positive intent. Instead they seem to be priming a superiority trip.

[theonething]

Agreed.

Or they could said:

"Be kind because it's the right thing to do."

[atmosx]

This. This is good enough reason to be kind. So let’s do that :-)