I've been thinking about these things a lot lately too, as an only child with older parents. It can definitely be tough - especially the guilt that you mention, of worrying what will happen if you're not nearby when they have a medical emergency, or what if something happens to you, who will be left to care for them? Ultimately I wouldn't change things either, but the yearning that you describe for another person to not only carry that burden but also share family history with is still there, sometimes.