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.
Nobody will be happy losing, all money spent no matter how little was worked for and deserves to be utilized properly. Gambling with what you can be able to lose, does no justice when the gambler repeatedly lose money, losing consecutively will become a problem and most people appreciates taking a break out from gambling for some time, maybe returning again will give them hedge over their casino or sports book. If the little amount spent on gambling for long days of losing, accumulatively in days and weeks will amount to a big figure.
Correct! and its normal people reaction when we are losing since no people would really say they are still happy despite of losing. Since provably that people would think about its a waste of money for losing such amount especially when they think there's some mistake has been done, they can avoid to lose if they do more better approach the way how they handle their bets.
This regrets would always come up, but we can always get over with those losses that's why gamblers continue to gamble despite of nothing is really sure and they don't know what's coming up if they play again for another round.