Harry Potter had been a child who grew up dependent on others, surrounded by his family's disgust and the exclusion and bullying of his peers.
He had gone through things similar to what Snape had once experienced.
So he had always deeply empathized with Snape's suffering.
When he experienced those memories through the Pensieve, he could truly feel what Snape had felt after being bullied for years by a group led by Harry's own father.
Whenever he pulled his thoughts back from the Pensieve, tears would always slide uncontrollably down Harry's cheeks.
They were echoes left behind by an immense amount of pain.
Some of that pain came from Snape, and some came from Harry.
After seeing so many beautiful memories of his parents, how could he not suffer when that dream was shattered and he discovered there was another side to everything? Originally, after Snape had targeted him with constant sarcasm from the moment he started school, Harry had despised Snape.
He thought Snape was gloomy, frightening, prone to picking fights for no reason, and extremely biased toward Slytherin.
Even after Roger had shown him a future involving Snape, Harry still harbored an extreme prejudice against him.
But.
After personally experiencing Snape's suffering and the feelings he had felt back then, Harry's attitude toward Snape changed from disgust to guilt.
Snape's hostility?
After going through all that pain and having the only "ray of light" in his life taken away, Snape had merely mocked him verbally. That was already generous enough to be called magnanimous!
Now, Potions class had become the most terrifying class in Harry's eyes, because there, he had to face someone he did not know how to face.
It was different from the fear he had felt in the past.
Now, his heart was tormented every