Every birth is a gamble. A soul enters the world innocent and pure. But it may not stay that way. This world is a maze of diverging pathways, both good and evil, and the choice is ours which way we go. Once a soul enters a body, it is free and therefore vulnerable to corruption. While acts of good elevate the soul, every act of evil makes a blemish on the soul.
Some souls are so lofty, it simply isn't worth the gamble. These souls are too precious to risk being compromised by life in a body. They are too high to come down to this world. But the other option, not to be sent down at all, to never reach this world, would mean that we would miss out on meeting these holy and lofty souls and hearing their message.
So these souls do come down. But in order to be protected from the potential evils of an earthly existence, they are sent down into a body that will not compromise their holiness. They enter this world in a form that is above sin, above evil. From a purely physical perspective we call them "disabled" or "handicapped"; from the perspective of the soul they are protected. They will never sin. Their sojourn in this world is often brief, and in terms of this world may seem sad. But they have retained their purity. And they have fulfilled their mission.
These special souls remind us that true love doesn't need a reason. We often love others for what they give us -- we love our children because they are cute, smart, and high achievers; we love our spouse for the pleasure and contentment they give us; we love our parents because they care for us. This is love, but it is not pure.
When a child is born that will never achieve worldly success, cannot provide the usual source of pride for his parents, all extraneous reasons to love him fall away and what's left is the purest love that there can be. These children are lovable not because of what they do for you, and not because of what they will one day become, but simply because they are.