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.
It also depends on how you are in gambling mode, I also often have gambling days and the reward is simply ice cream, there can be many winning days and sometimes full of losing but the smile is always here when given with great friends. On the contrary, people in the responsible mode always just know how to rush into a hellish game and then complain about responsibility and disappointment, they have not defined the exact gameplay and responsibility, they only know one bad side and do not change the other mode with gambling.