Will repeat OP question but in other words: "Who would be happy when he is losing?" Really, who would not feel disappointed after losing or failing. I believe there is no one who will shout "HOORAY!!!" when he has lost any size bet. In addition to what is already said, that it is ok to be disappointed as we are humans and express different emotions, it is normal to lose in gambling. Impossible only to win, a life can not consist only from victories or happy moments.
Correct, I think the confusion on OP statement derives from the frequent saying on gambling that we should play for fun only and consider our money lose when we play that’s why many think like OP which the feeling of disappointment is not okay when they lose.
Even millionaire and pro gambler feel upset when they bust their bankroll on a bad lose because it’s a normal reaction of a person when they lose money.
There’s no way someone can play with completely no emotion at all.