I actually have long disagreed with this sentiment -- I know people who don't want and never want kids, and they seem to totally comprehend the kind of love I have for my children. I also remember being a teenager and always kind of understanding the depth of the love my parents had for me (as in knowing "they love me implicitly, and they'd die for me, and they wish me so much happiness", and all the other trappings that I'd use to describe it with my own kids).

Being able to feel that love directly is, of course, a different story that's probably indeed pretty unique.