Forty years ago you and your brother foughts about who would get to play with the new train. You remember the fight, but not the outcome. At the time, you really wanted to win the fight and play with the train. Now you no longer have any strong preference for having been the one who got to play with the train.
In the view from distant hindsight, we do not have strong preferences that we were the ones who obtained some good—at least if we are talkiing of material goods. Now Simone Weil thinks, and I think common sense supports her, that the view from distant hindsight is the true and objective way of looking at a situation. Since it is not rational to see it as strongly preferable that we had long ago received some material good rather than someone else getting it, neither should we have such a stroong preference in regard to near-future goods. This is an argument for altruism and against egoism.