My reaction can be mild to frustrated. I know the odds and all the possibilities, but I can be upset with someone chasing their High Card Ace to the river, especially if they entered the pot out of position with Ax offsuit. Then when they call to the river with nothing connecting on the flop or turn, I can be quite ticked off. But, I know the long game is more important than a single hand.
Many of my friends have a very large range of opening
hands, which, in the long run, means they are losing players. So, when I am holding KK, or QQ and see an Ace hit on the river, I fully expect that one or more players still in the pot has connected and got me beat.
The only thing anyone can do is play the next hand, and the next...
If we understand the mathematics of poker, we know what can happen and just hope the math falls our way.