out of all the metaphors the author could've used. why not just go with a gardening example? friendships grow like trees, they need tending and they grow stronger with age.

especially because we're all familiar with friendships that turn out to be purely transactional relationships like with drinking buddies which fizzle out as soon as the activity itself is no longer welcoming. treating friendships literally as investments encourages surface level transactional relationships, which is the opposite of what you'd want to be happy!