I wonder if it would be more accurate to say that the love for a child is the closest two people can get to experiencing unconditional love - one on the receiving end and one on the giving end. It is not guaranteed (as the staggering numbers of child abuse cases showcase) but it is a possibility.
Loving a spouse unconditionally does not appear to work in the long run, simply because at some point in time the person will cease to live up to our expectations. At that time it is the conscious decision to keep on loving in spite of that person's character that keeps the relationship alive and thriving.
Perhaps it is the fact that in a marriage there are well defined roles each partner is supposed to play, while a parent-child relationship is one-sided in the role department where the person with the responsibility for acting does not expect anything in return.
