I think this varies much between the individuals. The relationships and personalities.
For myself, I absolutely want to know as much as possible. Especially the good or exciting parts. I can easily drown in compersion and the excitement can be contagious. Much like your analogy of the best friend etc. But that's the type of relationship my mate and I have. And it fit's both our personalities.
But I think not all relationships are like this. Some people seem to need that something they can claim as 'all theirs'. I understand and respect that. We confront such issues directly. The rule being......if there's something you just need to be sole owner of - just say so. Just say you really are uncomfortable sharing that. And if I hear that - I let it go.
But before you can reach that level - the other person has to experiment with sharing SOME things and have the chance to see that your compersion is indeed real. The more they see this, the more they are likely to share, as there's nothing like both being on the same cloud !
Like both hitting winning scratch tickets.
Start small, with small things, and try to work towards the bigger things. MAybe you'll end up sharing it all like we do - maybe you'll only share certain pieces. Whatever the case, celebrate what you share and RESPECT what you don't - as long as it feels safe for everyone.
My thoughts anyway.........