Your adrenaline is spiking. You breathe more heavily because your racing heart needs more oxygen. Your face is probably flushed, and your pupils dilated.
These physiological symptoms: The result of aggression and intimidation, or passion and attraction? Well, it could be either. The keys for the two are—visually at least—very similar. They're both caused by adrenaline, and how you interpret them depends largely on the content.
This is well demonstrated in the Wicked song "What Is this Feeling?" which was written to be an ironic parody of love songs.
My head is reeling,
My face is flushing!
What is this feeling?
Fervid as a flame
Does it have a name?
Yes...
...loathing!
This is especially evident in visual mediums (i.e. TV and movies) while it's less noticeable in books. In books, the above-listed physiological symptoms may or may not be mentioned, even if you could infer they're present. If it's a tense scene and the author wants to keep the action moving, slowing down and describing their body language might bog it down, so that's a good reason to skip over it. And you've usually got a narrator, reminding you how scared this character is of that other character.
In visual mediums with actors, though, it's... different. The physiological symptoms will all be there because that's what acting entails. Couple that with the fact that there's probably no narrator to remind you that this is caused by fear. If they're trying to intimidate each other, add some staring and invasion of personal space... maybe some Witty Banter too? No wonder this seems USTy.
And other tropes, such as Belligerent Sexual Tension and Villainous Crush—which other stories have long-since taught audiences to look for and recognize—can compound this impression. It's easy to get... confused.
