While the exact origin of the "Thanks, I hate it" phrase is unclear, its usage saw a spike in the fall of 2017 not only on Reddit but on Tumblr as well. According to the Internet culture database Know Your Meme, it was popularly used in comments responding to strange images before it was used to title such posts.
People began using the phrase in their post titles somewhere around late 2017 and early 2018, so you can say the subreddit r/TIHI (created on November 09, 2018) is a way of keeping this meme alive.
Mashable writer Heather Dockray believes that "Thanks, I hate it" is a celebration of what they call Internet Gothic; the grotesque consisting of hellish images, perverse Photoshops, photos suggestive of late capitalism, disturbing GIFs, shitposts, nihilist memes, and all the stupid stuff we try and forget we saw on the internet.
If it wasn't for r/TIHI, the meme might've died.
"The initial idea came from another moderator of us (u/scrumbly) on another post on r/ATBGE ... He posted a comment linking to this sub and as it didn't exist yet, I decided to make it a thing," Markus, the founder of the subreddit, told Mashable in an email. "I wanted to create a place for all the beautiful posts that are both likable and hatable at the same time. But I wasn't all that serious about it, as all subs I managed before were very small as well."
This one, however, lives on. And flourishes.
