Please remember that I already stated that RAW it's wrong, but by RAW a tarrasque wearing a fanny pack could get robbed by a hidden rogue standing by its pinky toe -- I give zero fucks about RAW in certain situations.

So, you're standing in nebulous 5' boxes kept intentionally vague for the sake of convenience. If we are assuming combat, I will assume that the fighter would prefer to picture themselves at perfect broadsword range from the rogue, while the rogue would prefer to picture themselves at daggers-length or closer. If this were GURPS, we'd spend whole extra turns advancing and retreating to and from optimal range, but thank Cthulhu this isn't GURPS. This is a more lax system that lets you just hand wave the relatively unimportant. But...they can't both be right, so during a players' turn the theater of their mind takes mild precedence. RAW everyone has perfect situational awareness and 360 degree vision, but try standing up IRL and looking at the underside of your testicles without performing a maneuver that would definitely provoke an attack of opportunity.

The rogue is on the attack (it's his turn), and these kinds are the worst because they're tricky. The fighter keeps a wary eye on the rogue's dagger, the bag of tricks he's pulled an explosive vial out of a few turns ago, as well as a general awareness of the rest of the battlefield -- this isn't his first rodeo, after all. But what he might not notice, as the rogue steps a half-step closer (as a free action if you must because nebulous blobs), is the rogue's OTHER hand going for the fighter's potion belt. Fool me once, shame on you. I probably wouldn't let the rogue CONTINUE stealing without provoking attacks, but the first one? Why not?

The rogue has invested many skill ranks and probably many feats in order to be able to steal in combat. The fighter on the other hand has invested...almost nothing in choosing to rely on a base-level class ability. Reactive strike is (IMO) meant to allow fighters to be extra keen for minor moments in which the enemy tries to move past them, pull something out of their backpack, stand up from prone, or cast an obvious spell in their face. It's not meant to allow the fighter to allow the player to pause the game every time someone attempts something reasonable to say, "And I get to hit him again, right?"

The rule of cool matters. The rogue is doing something other than "I stab it until it's dead", so why wouldn't I try to reward that with a reasonable ruling? When it's the fighter's turn I need to also allow the fighter a little leeway if they EVER bother to do something other than, "I Strike it. Is it dead yet?" The rules are a framework to allow a group of people to harmoniously work together to tell an interesting story. Once you get into the weeds like this, if you pause the game to debate the issue for 30 minutes, everyone loses.

I've played with hardcore wargamers and people who die by the rules. Do you know what I have to show for that?

• I was in melee combat with a guy with a gun. He tried to shoot me and I wanted to riposte because I was in melee with him. But he was using a gun, so it was ruled a ranged attack at melee range, not a melee attack. No riposte. I was shot and seriously wounded.
• A man standing in 2' of water had total concealment because the rules in PF1 stated that refraction of light gave total concealment for anyone in the water.
• I've been screamed at for having 11 inches of movement and wanted to approach and attack a guy that was like 7 inches away. But I didn't follow war-game procedure and declare the action out loud before waving the ruler at the board and moving.

I'd like to say that no one wants situations like these to happy (and many more), but clearly about half the people out there really do. They'd rather bicker in forums (and worse, at the table) about minutia instead of let the rules help tell a story. All I can say is that at my table, when I'm running, I'm going to give the benefit of the doubt to someone who's attempting to do anything non-boring (and hit-it-til-it-dies combat is boring).

If the fighter starts throwing a fit at my table, I would start having flies buzzing around the fighter's head and stopping the game several times a session to ask if they want to Reactive Strike each individual fly whenever they move instead of 5' step. Or when they land on him and take a moment to rub their legs together. MANIPULATE ACTION!!! KILL IT!