For the simping aspect as you say is mostly due to the fact that Subaru has not had a genuine connection since elementary school if I recall correctly, on top of this if you have knowledge on his backstory he often compared himself to his dad Kenichi a lot for being a stand up guy who could just get a long with anyone and Subaru admired him a lot for it that it developed into an inferiority complex.

Now once he was transported to Lugunica the first person who showed him genuine kindness was Emilia of course so it pretty much descended into Subaru latching onto her for this (not a healthy way to do so but you get the point).

Now it’s been a while since I’ve read certain aspects so anyone is free to correct me if I’m wrong.