Oh, so I guess Raimi's Peter wasn't self-centered and petty when he decided to stop being
Spider-Man, potentially allowing dozens of people to die, and willingly let's a guy get mugged by not trying to do anything at all. He realizes he was wrong for it and decides to be a hero again.
Why can't Peter do the same for the Gwen situation? He made a bad decision and realizes the impact of it, so he decides to improve upon himself. Also, the clerk said "What, your mommy didn't give you your milk money?". He's an orphan who just got into an argument over his parents with his Uncle. You telling me that it's unreasonable for him to be p*ssed off by that and possibly take it personally? Btw, if you remember correctly the robber threw Peter the milk, which probably made Peter even less wanting to interfer.