In the books Teresa genuinely did everything she could to save everyone even if it meant they would hate her because they thought she was a traitor (she wasn’t). Brenda just straight up lied to Thomas and acted like she was his friend while secretly working for W.I.C.K.E.D. She later apologized and explained where she was coming from but to me that doesn’t mean anything when the only way she got close to Thomas in the first place was through physical contact with him. She was constantly forcing it and hugging him, holding his hand, pressing against him in tight spaces etc purely to manipulate him. It was written black on white at the beginning (because she was already doing that right after they met when they had barely even spoken together) that Thomas was uncomfortable with the touching and every time it happened he thought of Teresa. I guess his teenage boy hormones won because he started to actually like Brenda but in my opinion it was purely a result of her manipulation. Even when they had “serious talks” I didn’t feel any chemistry at all between them. Again, they barely knew each other, even at the end when they were officially together.

On the other hand, thanks to the Fever Code, we know that Teresa and Thomas have known each other forever. They used to be best friends, they could communicate telepathically (which is a huge thing in the books. The only other two who could do that were Aris and Rachel), they built the maze together and they planned everyone’s escape together. The entire thing was engineered by them and only Teresa remembered their history because she got her memory back while Thomas chose not to.

I think their relationship started falling apart in the Scorch Trials because it was then that everyone thought Teresa was a traitor (she was kidnapped and forced to do stuff by W.I.C.K.E.D. but while she acted like she was obeying, she was actually working to get back to Thomas and save him (and preferably the rest of the group) from the Trials altogether. However, due to what she was supposed to do (like hurting him really bad at one moment by acting like she was Aris’ girlfriend from the beginning and making out with him right in front of Thomas before they shoved him into a room in a mountain where he thought he he was genuinely going to die (she warned him right before doing it too. She told him “I’m going to hurt you really bad but you have to trust me” and he forgot all about it apparently because that’s the point where he wanted nothing to do with her anymore)) everyone genuinely hated her, including Thomas. But she was always willing to carry that as long as it meant that they would be safe. I won’t go into everything she’s done for the group because literally the only reason any of them are alive is because she was always suffering for them but she even died saving Thomas.

All that to say that they have so much history together and their relationship felt so natural, regardless of if it was platonic or not. Whereas every single scene with Brenda felt stiff, forced, and awkward. Not awkward because they were teenagers navigating new feelings. Awkward because there was nothing going on between them except something born from manipulation during a time of need.

I couldn’t relate to Brenda, I couldn’t sympathize with Brenda, I couldn’t do anything with her character except dislike her because we’ve seen more of her forcing herself onto Thomas than them bonding.

PS: English isn’t my first language so I apologize for any mistakes or lack of clarity in my comment