The readership seems to be divided equally on the question of Voldemort and love: the first half holds onto the opinion of: “Don't question the author, stupid”, while the other half appears interested in exploring all sorts of possibilities. I'm firmly in the second camp. I choose to question. And here's why.
Few would dispute that love is integral to the human experience. Love and affection, care and attention are indispensable to a child's upbringing. Remove those, and there would be some very serious consequences. Children are born with the need for love. If nothing else, it was essential to their survival in ages past - so much so, that it's probably our nature.
Now imagine a situation where this need for love and the ability to feel it are removed by applying an influence from the outside. Imagine that something as simple as a parent using a love potion can rob an innocent, unborn child of an integral part of their soul.
JKR had created a masterpiece that deals with the most fundamental concepts of good and evil. The ideas of evil that she had created are horrific indeed: Dementors, Inferi, Horcruxes... Few realize that with just a few words she had also created an idea of Evil so profound that it makes all of the above seem a child's play in comparison: an idea that there is a very simple way by which someone can destroy an innocent child's soul.
If we take for granted the idea that in the magical world, a child conceived under the influence of a love potion is damaged, then this is Evil beyond imagining. In this case, any person who even thinks the words “love potion” (never mind brews it or uses it) should be subject to the Dementor's Kiss on the spot. A punishment to fit the crime – destroy a soul and you'll lose your own. (Since this is not the case, and smart wizards like Dumbledore and the Love Lab in the Department of Mysteries would've figured out the damage by now... well, one more argument against the love potion damage theory, isn't it?)
I don't believe for a minute that Lord Voldemort was the only child ever conceived under such circumstances. The use of love potions seems widespread in the Wizarding world - it is even available to underage witches and wizards! But Voldemort is the only one who had become the epitome of evil. So, either the other potential damaged children are all in hiding, or there were other factors that influenced Voldemort's development.
There's also the issue of the author stating that had Merope Gaunt survived, her child would potentially not have succumbed to evil. This is a direct contradiction. If he was born without the ability to love, his mother's love, life, or death would make no difference. None.
I also have another argument. JKR had created a magical world so believable that we can practically taste it. But the factors that had made her books the worldwide phenomenon they are today are not all magic; it's the characters, as well. The characters and the choices they had made – often between what was right, and what was easy.
The worst thing that could happen to a book is a one-dimensional, flat hero. The second worst thing that can happen to a book is a cardboard cutout villain. Thus, if we apply the theory of Voldemort's innate inability to feel love, isn't “He was born this way” the same as stating “He had no choice. He was born evil, therefore he was destined to succumb to the evil within”? And if we take away a character's choices, motivations, influences, etc., what have we got left? A power in the world that is evil because it is. A cardboard character.
If Voldemort had no choice, and Harry did, this brings the situation from bad to worse. It's no longer trust, friendship, and love vs. fear, greed, and a quest for power at all costs. It's a situation where the hero triumphs over a villain who had no chance for redemption, who had been programmed by his very circumstances to act as he did. Does this not devalue somewhat all the sacrifices Harry had made?
So if you would say “Why question the author? I choose to believe that Voldemort was born unable to love”, consider this: the “inability to love” idea is not only horrific, but also destroys the moral, the very fabric of our most favorite story. It's our choices that make us who we are. And a villain who had no choice has no place in this story.
Harry knew that there are always choices. Otherwise he would not have offered Voldemort to try for some remorse.