system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] Why couldn't Harry see the Thestrals after his first year?

And maybe even sooner form the beginning?

He saw his mother die as a baby, and at the end of the first year he saw Quirrell die right in front of him!

This are at least two deaths he witnessed, one when he was a fully conscious functioning human, I can accept that seeing death as a baby doesn't trigger Thestral-vision but surely seeing the guy you just hugged to death, die should count?
gpt
In the books Harry was unconscious when Quirrel died so he wasn't able to witness it. 

In the movies it could be that the death has to be traumatic to a certain degree to enable someone being able to see Thestrals. It's possible that Quirrel's death didn't impact Harry enough to enable him seeing the Thestrals. Alternatively, maybe because of the circumstances Harry wasn't able to fully process the death as he passed out mere moments later.