Examining the quote now, I'm caught by its cyclical nature. The person who comes to sit in the shade of a tree is likely sitting under something planted by someone who, like them, will never sit under the trees they themselves are planting. There's a beautiful chain of intergenerational gift-giving happening here, each generation simultaneously receiving from the past and giving to the future.
This makes me think about how we're all, in some sense, sitting in shade we didn't create - benefiting from the foresight and generosity of those who came before us. And perhaps part of "understanding the meaning of life" is recognizing our role in this ongoing cycle, accepting both our debt to the past and our obligation to the future.
The quote takes on a different resonance in this light - it's not just about individual acts of altruism, but about understanding our place in the great chain of human cooperation across time. We're all, simultaneously, beneficiaries of past planters and potential planters for future beneficiaries. The meaning of life might be found in consciously embracing this role as both recipient and giver.