When my sons were young as I'm sure is the case with most kids, they also hated to lose and it played havoc with their emotions at times. Gaming was a large part of them learning to win and lose gracefully in addition to emphasizing the following often:
1. You learn more when you lose than when you win because most of the time the person who's beating you is better than you at what you're playing. You learn little when you're handily beating someone. Never quit in such a scenario, you're passing up a prime opportunity to learn and to learn how to work your way out of a difficult scenario.
2. It would get boring winning every time just like it would get boring if all you ever ate was ice cream. The challenge is half of what makes a game fun.
3. There is room in the world for more than one good player, one smart person, or one highly creative person.
4. The sun will rise tomorrow, I'll even prove it to you by reminding you first thing in the morning. You'll even find you enjoy at least part of tomorrow.
5. Show you child someone throwing a fit when they lose and ask them how they view that person at that time. Then ask them if they think they appear anymore appealing when they throw a tantrum when they lose.
6. Teach them to turn losing into a positive thing by letting it inspire them to practice harder / learn in more depth.
