Because after all, miniscule though every individual, every 'self', is, he/she/it is an object through which life is being expressed, and leaves some sort of contribution to the world. The majority of human beings leave their genes embedded in other human beings, other things they have made, everyone things they have done: they have taught or tortured, built or bombed, dug a garden or chopped down trees, so that our whole environment, cities, farmland, deserts -- the lot! -- is built up of contributions, useful or detrimental, from the innumerable swarm of selfs preceding us, to which we ourselves are adding our grains of sand.
I suppose you might tell from this passage I was feeling useless and pointless. From my perspective down here in downville, the idea that I might have dug more gardens than I have chopped is heartening.
To think our existence pointless ... would therefore be absurd; instead, we should remember that it does make its almost invisible but real contribution, either to usefulness or to harm....
(all quotes from paperback edition p180).
I suppose that pretty much sums up how I've been feeling: almost invisible but real... But I think I must remember to include the rest of the quotation. That is, if I can get to the end of most days having contributed more usefulness than harm, perhaps downville won't seem as down as it does at the moment.