Why am I my brother's keeper?

Lest I offend anyone else by doubting their worth, let me begin by doubting my own. I am not depressed; I think I have an adequate sense of self by standard psychological criteria; I think I am not deficient in ordinary self-esteem; I am certainly not deficient in everyday self-centredness and selfishness. And yet, if I ask myself in a cool hour whether I have some deep intrinsic ‘worth’ that grounds the importance of what happens to me, or that justifies anyone, myself or another, in caring about things for my own sake, I do not find it. Much that goes on in my life is important (in a small way); much of it has intrinsic value, both positive and negative. And those facts matter to how I should be treated. But the idea that they either depend on or manifest my personal ‘worth’ is what escapes me.