Sterek and Thiam mirror each other. They are very similar in dynamic and in how the characters are.
When I first watched Teen Wolf I missed in season 5 the dynamic between Stiles and Derek when Tyler Hoechlin (Derek Hale's actor) left the show. So when Theo is brought back in season 6, we get that dynamic again, but in Theo and Liam.
We get similar shots between these two ships, as seen below. And the only reason I have 5 examples is because Tumblr only allows you to add 10 photos to a post but I had more as well that mirrored each other.
I really do think they were trying to recreate the dynamic between Stiles and Derek with Theo and Liam.
I think they tried to recreate it with Stiles and Theo, but it didn't work. Their dynamic was good, but it was still different. Then they tried Theo with Liam and bam! Liam and Theo was much closer and worked well.
Derek and Theo have these things common
1. They lost their older sister, who haunts them, even if in different ways.
2. They were groomed as minors, Derek by Kate and Theo by the Dread Doctors.
3. They were from Beacon Hills, left, and then came back when older.
4. They're the broody of the ship but they know how to act charming.
5. They're suicidal, beyond traumatized, total martyrs, and the darker one in the ship.
6. They did many things they weren't proud of to get a pack. They made mistakes and paid high prices for them.
8. At one point they were homeless.
9. As far as we know, neither of them graduated high school.
10. They lost their families and were alone.
11. They get super big noticeable heart eyes for the other person in their ship. Like seriously they both look adorable af with the heart eyes.
12. They can pass as straight, but they definitely aren't. (I personally believe Theo is demisexual and Derek is pansexual.)
Stiles and Liam have these things in common
2. They antagonize the other person in their ship.
3. Have severe mental disorders that causes them to lash out, usually in anger.
4. They cling to their best friend to the point they could be dating. Like they are platonic soulmates with their best friends (Stiles with Scott and Liam with Mason).
6. They are the more dominant personality in their ships.
7. They tend to get physical when angry.
8. They are the sunshine in their ships but they also have their dark sides.
9. They take care of others and not themselves. They put the needs of their loved ones above their own needs.
10. They have to hide the supernatural from their families and keep secrets which makes their lives harder.
11. They have chaotic disaster bi energy. (I personally believe Stiles and Liam are bisexual.)
12. They always go back for the other person in their ships, even when they don't like them.
Both ships are perfect for my personal favorite trope, enemies to lovers.
Both ships are filled with snark and pettiness.
Both ships have threatened to leave the other in their ship behind or not help them when they need it, but it's an empty threat, and they both know it.
Both ships are forced to work together and they learn how to get along and gradually become friends.
I'm honestly surprised there aren't more people out there who ship both Sterek and Thiam because of how similar their dynamics are.
And obviously ship and let ship. This post doesn't mean people are required to like the other ship. I just think these two ships have been at war for so long that it can be hard to fathom liking the other ship because that's the other ship.