Inevitably, in the few days since, the subject has come up in a number of ways in our house. In the first instance, Charlie demonstrated that the joy of helping can always be a source of tantrums. Ben helped Elissa and Charlie missed out on the helping opportunity, resulting in a melt down. If I don't find garbage in pairs (one for Elissa and one for Charlie), the whole day can go to pot.
More usefully, last night Ben volunteered that at school they are always told that it's more fun when you share, but he was definitely less than convinced that this was true. He gets that he needs to share anyway, but he feels it as an obligation, not a source of joy. After a discussion about the fact that even as an adult, there can be certain things that one shares out of obligation (there may be a few kinds of chocolate that I share only out of obligation, but good wine I love to share), we started talking about sharing toys, and about serving communal food around to everyone and about how it makes you feel good to make someone happy. Ben was delighted to discover that there are lots of occasions when sharing and helping do make him happy - he's just been focussing on the pain of sharing the small bag of lollies with his siblings.
So maybe a big part of teaching altruism is just pointing out when it's happening already.