The Benefits of Being Kind

There are times when I truly feel that being kind and gentle to others is, in the end, for one's own sake.
I remember hearing on TV a while ago that the meaning of the proverb 'Kindness is not for others' has changed over time.
According to that, it used to be used to mean that showing kindness to others would eventually come back to you, but now it has changed to mean that showing kindness is just spoiling someone and does not benefit them.
In other words, it has recently changed to mean something like, 'Don't show kindness to that person because it won't do them any good!'
It felt a bit sad, as if it reflected a harsh world. If you interpret it even more broadly, it almost feels like it's encouraging us to think only of ourselves even more.
While thinking about that, I suddenly realized something.
Looking back on my daily life, I noticed that when I am treated with kindness, the hurdle for being kind to someone else afterward drops significantly.
The other day, when I was trying to get off a crowded train but couldn't, the people around me stepped off for a moment, and I was able to get off without incident. After experiencing that, when I see someone else in a similar situation, I find myself stepping off the train and doing the exact same thing.
It's just a simple scene from everyday life, but I think that's just how humans are.
If someone glares at you, you want to glare back (though it's sad because sometimes the other person just has a naturally mean look or bad eyes...), and if someone yells at you, you want to yell back.
Especially in the workplace, people who are always being scolded might have anger built up without realizing it, so I think controlling emotions is difficult.
We are not gods or Buddhas, so we aren't noble enough to force ourselves to be kind to unpleasant people, but I can't help but feel that the memory of being treated with kindness is ultimately the driving force behind being kind to others.