Why Do We Hold Doors for People?

Why do we hold doors for people?

Maybe, just maybe, simply to get a 'thank you'

Those words on their own hold little weight,
they offer no reward or value
as words are just sounds until we attach a meaning to them.

So what reward to we reap when we hear that simple 'thank you'?

We receive not praise or a pat on the head but the acknowledgment that, no matter how small or simple, our act of kindness was appreciated.
The recognition of kindness, for kindness' sake....

That, I think, is why we hold doors open
for random strangers