How to best share joy depends on the child. What they're delighted about. And what seems "authentic" (meaning sincere and honest) to you.
Let's say your child comes home bubbly and joyful because they made a friend at school. What would you say?
A. "That's nice dear." [and then turn away to do something else]
B. "Oh my goodness, that's amazing! That's just about the best news ever! I knew you would do it!" [in a squealing voice, jumping up and down in excitement]
C. "That must feel really good. I'm so happy for you. I remember you were so worried about meeting a new friend. Tell me about how you met!" [stay with, smile, make eye contact, and engage in a conversation to hear their story]
My ranking for the above three responses? C would be the best. And if I had to choose between B & A, I would choose B. Why?
C is about "being with". Getting into the story of their emotional experience. Cause it's all about the story! That's where the understanding and the making sense of and the experience of connection with the other comes in. That's when they experience you "being with them" in their joy.
B is about "celebrating" which does offers opportunity for shared experience of joy, but without getting the story you don't get to communicate that you really see, hear and understand what your child is going through.
A is "indifferent". If you've given an "A" response, try reading this blog post and give yourself a second chance.