Your arm goes out before you decide anything. Someone stumbles on the bus, you have already steadied them, and only afterwards does your brain file a report about what just happened.
That reflex is older and faster than deliberation. It runs on the same equipment that makes you flinch at a slammed door.
What slows it down is rarely coldness. It is calculation: whether it is your business, whether they will be embarrassed, whether you will look strange. The instinct is intact underneath all of that. Most of the time the kind thing is simply the first thing, arriving before the second thoughts do.