Many people say they desire unconditional love. They want to be loved “forever and always” - they want love that lasts, no matter what happens. Or at least, many people say they want this. I will argue that, upon reflection, you will probably find that you don’t want this.
Let’s contrast unconditional love with conditional love. Conditional love is love that is conditioned upon the beloved retaining some properties. If your love is “conditioned” upon the fact that your beloved is pretty, then your love for her will only last as long as her prettiness. We often speak of “conditional love” as shallow and vain for this reason. It need not be conditioned upon such fleeting factors, however; one’s love for their partner could be conditioned upon their virtue, for instance.
Unconditional love, on the other hand, persists no matter what properties the beloved gains or loses. Alternatively, we might say that unconditional love is love that is conditioned upon those properties of the beloved that cannot be lost (their essential properties). A love conditioned upon essential properties may still warrant the appellation “unconditional” because it will never cease.
It’s common for spouses to ask each other why they love each other. “What do you love about me?” - a good question. If your partner’s love is conditional, then they can answer your question easily. “I fell in love with you because I found you smart, charming, beautiful, and virtuous” - or something to that effect.
If you partner’s love is unconditional, then they will have a much harder time answering the question. That’s because their love is not conditioned upon any property you possess - not your looks, personality, or values. No matter how funny, smart, or pretty you are, they would love you just as much if you became evil, stupid, grotesque, and dull. Alternatively, they could say that they love you because of one of your essential properties - the fact that you are numerically identical with yourself, or the fact that you are so-and-so’s child - but we would be disappointed if someone said they loved us for those reasons. We want to be loved not just for any reasons whatsoever, but for reasons that seem to warrant love.
Think of it this way - people have characteristics, or properties. We can divide these properties into two categories - properties that are non-essential (these can be lost), and properties that are essential (these cannot be lost). The trouble is that all of the properties which seem to warrant love - all of the properties which serve as good answers to the question “why do you love me?” - are non-essential. Prettiness, funniness, virtuousness, intelligence, playfulness, and so on, are all non-essential. People can lose these properties. Essential properties never fade, but are the sorts of properties you would never invoke during an explanation of why you love your partner. and you wold be dismayed if your partner invoked them in their explanation of why they love you.
The alternative to love being conditioned upon a property is for it be conditioned upon no properties. A lover like this would have no answer to the question “why do you love me”. There is quite literally nothing about the beloved that could possibly explain why such a love exists. It is entirely arbitrary.
The best counterexample to the case I’ve sketched here is that of parental love. It seems that many parents love their children just because their children are their children - it is an essential property of their children that they are their children. Thus, parental love that is conditioned upon this essential property will never end. If your reflect on this sort of love, however, you may find it just as puzzling as the unconditional love criticized above. Harry Chalmers has a nice essay exploring this topic.
A person is my child if I cause them to exist through biological reproduction. We can ask: why should I love someone simply because I caused them to exist through biological reproduction? Imagine that my neighbor creates a child that is qualitatively identical to my own. Why should I love my child instead of theirs if the two children are qualitatively identical? Facts about the origin of a person - of the cause of their being - do not seem to have much to do with love. I can cause lots of other things to exist besides children, and would not thereby have a reason to love those things simply because I created them.
Even more puzzling to me, is this: the fact that my child bears the essential property of being my child is supposed to gives me a reason to love my child but not other people. Why should this fact only matter to me? The fact that Sally is my child apparently gives no one but me a reason to love Sally, but why is that? I do not see what further explanation is available here.
Maybe I have gone wrong somewhere here - I acknowledge that I advance a counterintuitive and potentially disheartening position. If this is the case, I would content myself with the concession that parental love is a special case, and advance the argument that unconditional love is not desirable in the realm of romantic love.
There is a conflict, then, between our desire for love to last forever, and our desire to be loved for reasons. Love is only guaranteed to persist through all the changes we undergo when it is either entirely arbitrary and hollow, or conditioned upon properties that no one actually cares about. We want to be loved for the right reasons, but this means that love is not impervious to the vicissitudes of fortune. The sweet spot, it seems, is love that is conditioned upon properties that are study, durable, and relevant to an attitude of love. Such a love can perhaps make up for its mortality with its meaningfulness.
But it seems like origin features can be a good basis for relational properties. Then, relational properties can act as proper reasons for love. In the duplicate child case, I think the mother has no obligation to the duplicate, because she has no shared history with them to properly justify the relevant type of relationship which in turn acts as a reason for love.
I think there's a difference between why we fall in love with someone and why we continue to love them. Perhaps I was drawn to someone because they were pretty and danced well, but that's just the reason I approached them and tried to win their love. Once we are deep in love, it won't matter if they get disfigured and lame; "for better or worse." Given a reasonable level of care, love can last for as long as we both shall live. It's not precisely unconditional, but it's close.
As for the new-born baby, the parents have had many months of anticipation, even if the baby has not. There is a history on one side, and that's the side that has love pretty much from the start. The new-born will grow to love its parents.
And unconditional love may not be common, but I don't think it's undesirable; it's more that it's not as satisfying as the target thinks it would be. (Romantically, at least. I think unconditional parental love is better than conditional; my teenage sons certainly put me thru some hard times that would likely have wrecked a romantic relationship.)