A friend's good news lands in two places at once, and most people will only admit to one of them. There's the gladness, which is real, and there's the small cold arithmetic underneath about where that leaves you.
The people who genuinely enjoy other people's wins aren't better characters. They have stopped running the arithmetic, usually because they got clear about what they actually want, and somebody else's promotion stopped reading as a subtraction.
It is the cheapest joy available. Other people succeed constantly. Being able to enjoy that is an enormous supply line for very little outlay.