That’s not how joy works.
Celebrating the victory of those you care about costs them nothing, even if you didn’t contribute to it, and your happiness does not come at their expense. On the contrary, it can only bring them more joy, like with kids when it comes to their parents.
It only becomes selfish if you claim that victory wasn’t theirs but yours. That they’d never have succeeded if it wasn’t for your help. That they should thank you for your assistance. And regrettably I’ve seen parents do this when it comes to their kids.
The idea that being happy for others (let alone yourself) is harmful or requires justification is dreadfully common among trauma survivors, and there’s little I can do than break the cardinal rule and tell them that’s not how they should feel and then find ways to show them why.
If someone you love wins, so do you.