I can easily let younger people win. Say, my five-year-old cousin, when playing chess. I had more fun than if I tried to win, mind you. I controled the game, just to create my own defat. I did win against my nine-year-old when I said I would really play my best, but then, I genuinely thought it could be a close contest, as I am very bad at chess. It was a massacre, but near the end, I gave her honest advice from time to time.
I don't think I would let someone win anything otherwise, unless I was trying to inspire someone into liking a game. I am very competitive naturally, but I actively try not to be, so I actively try to appear more detached. If asked, I will give honest advice to my opponents. I make an effort to congratulate my opponent for his good moves. I think it fails anyway, because my family rarely wants to play any game with me, saying I "always win", which is arguable.
I find people rather hypocritical at this. I don't really mind losing if I am having fun at a game and find it challenging, as happened when I was younger, but now that I win, no one seems to want to play...