I think they set it next to Rory's lecture to Lorelai to create a parallel between their post-breakup processes. Liz and Rory were telling Lorelai and Luke "You're always doing stupid things like this and losing people you love," each in the only way they could be heard.
Until that moment, Lorelai was trying to believe Christopher had nothing to do with her breakup and Luke was trying to believe Christopher was the only reason they broke up.
Their chance encounters show a similar parallel. Luke may have attempted to deny he was hurt by what she had done, but she attempted to apologize for it with a light 'I'm sorry' while passing him on a crosswalk first. She clearly wasn't ready to accept his anger, and it was to soon too volunteer it by apologizing.
The parallel breaks down at the supermarket when Luke has finally admitted that he can forgive (read: understand) why Lorelai had to act so rashly after their breakup. But he literally gives up on fighting their differences to be together, and this was clearly not the conclusion at which she had arrived. The hope in her face when he said he could forgive her and the disappointment she showed when he 'gave up' on L&L was more dramatic and potent than the 'I get the rocket! Oh... Christmas' scene. (Probably because it was in person instead of on the phone. They were both good scenes.)
It also made it seem like another 'now or never' moment in which he could have had all of her just by letting it happen. Whe he was yelling at Taylor for 'never listening to anyone and deciding what was best for everyone by himself,' he was really yelling at Lorelai. In the supermarket, he had a chance to decide.
Until the end of this scene, Lorelai still had hope. Sleeping with Christopher was an accident, and it happened while they were broken up when she was hurting and desperate not to be alone. If Luke could forgive her, she could forgive herself and they could pick up where they left off, determined to change together, blah blah blah long 'middle part,' happily ever after.
But literally nobody is willing to play along with pretending sex with Christopher doesn't matter (except Sookie, for some reason.)
Lorelai is fast. She thinks fast and acts fast and Luke would have to process a lot faster than he does to have any real input in her active decisions. But being with him has changed her and made her think more before she reacts to her feelings.
Luke is slow and independent and likes to be alone whe he's trying to think, but being with Lorelai has changed him and he's started acting on his own feelings by telling the people he cares about that he cares about them when he has the chance instead of fighting with them over their mistakes.
They both must feel like bumpers on each other's cars, but Luke never said that he was fighting anything to be with her before the supermarket scene.
Still, Lorelai loves him and also believes in love. If he'd fought for her by eloping (or just running after her) or even by pretending the night with Christopher didn't matter and kissing her in the supermarket (she's going home with me, Loser) she would have believed he'd just forgotten to unlock the door or didn't know how. The only way she would let go of the bumper was if he let go of the bumper of her racecar first.
One other thing - in the 'annoying' coversation with TJ (in the afternoon, after 'Kirk's' ad 'the crosswalk') TJ remarks repeatedly that Luke has 'quit drinking beer.' Has he? I can't remember (maybe why a character had to mention it for me to notice.) I thought it might be some foreshadow to Luke starting to drink heavily. Is it possible his big breakdown scene will land him in the hospital with alcohol poisoning from slamming back a case of Zima?
We can rebuild it. We have the technology. We have the capability to build the world's best Gilmore Girls forum. The ORG Forum will be just that. Better than it was before. Better, stronger, faster. 