I wonder if anyone has explored the idea that Petunia had a crush on Severus.
She resented magic and Hogwarts and people who had magic because she didn't. Her sister had those things. She wanted them, too. She wanted to be special, too. But, no. Lily was the special one, the pretty one, the magical one, the one who got to have a whole world where Petunia wasn't allowed to follow. Their parents' golden child.
She writes a letter to beg to please be allowed to at least attend the Wizarding school. But she was ignored. Denied. She wasn't special enough.
Then there was Severus Snape. The handsome, sweet neighbor boy who was Lily's best friend. But when she tried to spend time with them, she didn't always understand their conversations (often, they were about magic or Hogwarts or people who were at Hogwarts or about the Wizarding world in general). She paid attention but she didn't have the full context and they either wouldn't tell her or couldn't tell her. It was obvious she was a third wheel.
She only wanted to spend time with Severus, but he was only interested in Lily. Pretty, special Lily. Not ugly, ordinary Petunia.
And after a while she did what she always did. She turned the hurt into resentment to protect her own heart. But she took it deeper than most people might have.
And she pulled away from her sister and the handsome neighbor boy. She stopped trying to tag along. She stopped her awkward attempts to flirt and show interest in the things Severus was interested. The boy seemed totally oblivious anyway. Too wrapped up in Lily.
And it was plain to Petunia that while Lily liked Severus for a friend she was not in love with him and never would be. Severus was ignoring a girl who loved him so that he could chase after a girl who never could.
This just made Petunia more resentful. So, when she did have to be around them, she made sure to let them know how resentful she was. Not why, and without context. She thought that was fair. They gave her no context, she would give them none. She wouldn't give them the satisfaction of knowing how hurt she was.
She began saying rude and nasty things to her sister. Calling her a freak, making sure she knew that she wasn't special. Just freakish. Petunia was normal, respectable.
She made sure Severus knew the same about his own freakishness and that he was low class, anyway.
Petunia embraced her ordinariness, not out of a healthy conservation of her own mental health but out of pure spite.
Over the years, her spite and her resentment became real hate that allowed her refuse to attend her sister's wedding, give no indication she had heard about the birth of her son. Allowed Petunia to find the most ordinary man and marry despite the fact that he was a pig. And she gave birth to a most ordinary son and gave him a most ordinary name and lived a life she knew was as ordinary as she could possibly make it. She was a traditional, ordinary housewife and mother.
Who cares if she trapped herself? She was showing everyone that SHE was the true special one.
And when her sister died, horrifically, the level of her festered resentment and hatred allowed her not to care, to insist it served her right, and attempt to refuse to take in her own nephew, still a baby too young to speak or walk yet.
There was still enough of the original Petunia left for Dumbledore to manipulate into agreeing to take the boy in. But her hatred and her resentment allowed her to not only allow her husband to abuse the boy but to join in on it herself and teach her son to do the same.
And if, late at night when it is quiet and Vernon has rolled his bulk off of her after she had done her wifely duties for the night, she allows herself to cast her mind back to the girl she once used to be and the boy, now likely a man, whom she had loved, and the life she denied herself in the name of hate and resentment, no one but she has to know about it.
Please note, this is from Petunia's POV and as such it will be skewed. Regardless of who she was when she was young, she made her choices and did the things she did as an adult. None of this is an excuse for child abuse and neglect. That isn't the message or point of this.
This was entirely borne of me reading a fic where Snape goes to Privet Drive to check on Harry and calls her Tuney. It might or might not inspire more thoughts and prompts.
Additionally, I don't think Snape is mentioned enough in the right ways to tag him in any way, but this is NOT about Snape bashing or Lily bashing. I am 100% Pro-Snape.