My youngest son has a type of congenital myopathy that makes him very weak. While he can walk, he can't run or jump and falls down a lot. Needless to say he finds this very humiliating (he is 16).
We were at a restaurant and my son was standing next to me with untied shoelaces. Unfortunately he tripped and, due to his weakness, could not catch himself, crashing heavily to the ground in the middle of a crowded restaurant.
My daughters boyfriend, without missing a beat, immediately lay down next to him on the floor and asked him "How is it going down here," and otherwise made some small talk to ease some of my son's embarrassment.
He has always treated my son like his little brother but that selfless act was unforgettable.
Needless to say I have loved him like a son ever since.