The story of Voldemort’s mother is so tragic even Harry almost felt sorry for He Who Must Not Be Named when he heard it:
‘Merope refused to raise her wand even to save her own life.’
‘She wouldn’t even stay alive for her son?’
Dumbledore raised his eyebrows. ‘Could you possibly be feeling sorry for Lord Voldemort?’
‘No,’ said Harry quickly, ‘but she had a choice, didn’t she, not like my mother –’
‘Your mother had a choice, too,’ said Dumbledore gently. ‘Yes, Merope Riddle chose death in spite of a son who needed her, but do not judge her too harshly, Harry. She was greatly weakened by long suffering and she never had your mother’s courage…’
Harry Potter and the Half-Blood Prince
Merope might have proven a positive influence on Tom Riddle had she lived to bring him up herself. However, she died soon after his birth without trying to save herself with magic.
The suffering Merope endured at the hands of her cruel father Marvolo and brother Morfin left quite a mark. By the time the two men were locked up in Azkaban by the Ministry of Magic, the damage had been done.
She is described as so bleached of life that she blends into the grey walls around her:
‘Her hair was lank and dull and she had a plain, pale, rather heavy face. Her eyes, like her brother’s, stared in opposite directions. She looked a little cleaner than the two men, but Harry thought he had never seen a more defeated-looking person.’
After being left alone in the Gaunt family hovel, Merope bewitched the Muggle boy she had a crush on, Tom Riddle. But he would return to the village some months later muttering about being ‘hoodwinked’ and ‘taken in’ by the local tramp’s daughter.
Quite how Merope tricked the handsome and wealthy Muggle boy into wedlock is unclear but Dumbledore suspects a love potion rather than the Imperius Curse. How and why Riddle Sr escaped Merope’s enchantment and left her destitute and pregnant is also unclear.
Another of Dumbledore's highly plausible theories is that Merope was so in love with Riddle that she couldn’t bear to enslave him with magic long-term, and hoped he would learn to return her feelings.
Evidently this didn’t happen and Riddle returned to his family home without ever troubling himself to discover what become of her – to his cost 16 years later, when their son murdered him and the rest of the Riddle family.
Alone and destitute, Merope sold Slytherin’s priceless locket (yes, that one) to Borgin and Burkes for just ten Galleons.
It seems that when Merope lost her husband she also lost the will to live. When she arrived heavily pregnant on the steps of the London orphanage where Tom Riddle Jr was to spend his early years, she seemed to know she wasn’t going to make it.
Her dying wish that the baby should resemble his father was granted. She couldn’t have known that the second Tom Riddle would go on to inherit his father’s callousness, too.
It might be argued that Voldemort grew up devoid of love because his mother died for want of it, and that his father’s love was stolen rather than earned. Perhaps if he’d had any understanding of the difference between genuine love and the kind that you compel, Voldemort might have had a better grasp of its power.
Luckily for Harry, he did not.