I agree, I cannot understand how you can't just love any innocent child. I consider myself very lucky to have had a really secure childhood, it stays with you all your life and is the most important thing we can be given imo.
I don't think it's about being adopted, it's about the capacity of the adults, whoever they are, to be able to emote and give love, I suppose for soe if they don't experience it they have no way of knowing how to do it. 
Well, the strange thing is with my mother for instance, that she could love two of her children, but not the middle girl. Maybe some self hatred, I don´t know, because that sibling reminds the most of my mother. I can safely say about that sister that she has had it in her to give her son PLENTY of love.
We have another example in my family - maybe we are especially bad! Someone who has two girls and two boys. One girl and one boy get all the love, the other two, not so much. I can´t for the life of me understand it, they are perfectly beautiful, intelligent, pleasant kids!
Maybe this is very common, I don´t know, I hope not!