A reader notes:
Andrew, it wouldn't be a "mitzvah" to watch "someone you love blossom and grow and mature and thrive"! A mitzvah is a good deed, basically. Informally, your Jewish friends would use "mitzvah" to describe the helping of an old lady across the street or the donation of a kidney or the unrequested cutting of your neighbour's lawn. I don't know what they'd call what you're describing...in Yiddish, they might say you were "schepping nachas," which means, basically, to be filled with pride because of the accomplishments of someone else.
Schepping someone's nachas sounds pretty gay to me. So I'm in! What I meant, I think, is that it's a blessing, a mitzvah from God.