i think the reason (beside the proximity) why this specific scene makes all of us go so feral is how it illustrates how in tune with each other they are.
danny mentions the guys watching them, which, of course, steve already had noticed, because he's as aware of the circumstances around them as danny, don't let both of their relaxed postures and nonchalant attitude fool you, they are both good at what they do, danny doesn't have to explain anything because steve already picked up on it. and you know it's not a usual thing because they make the fbi guys look like unprofessional clowns. both steve and danny are impressive and they definitely noticed.
neither steve nor danny had that before, that feeling of having a true partner, the person who can fully keep up with you, work at your speed, with barely a word, that must be an invigorating feeling. they really just work. this kind of instant and strong connection doesn't usually happen. something tells me it really gets their blood pumping 😏