A good thesis for Sammy's epiphany in Updike's story should be a statement of how his life is affected by his decision to quit his job. When the teenaged Sammy acts impulsively in what he feels is a chivalrous act, he finds himself thrust into a world that is more complicated than he previously imagined.
Certainly, there is a new awareness of the world that impresses itself upon Sammy after he boldly quits his job to impress the girls who have immodestly entered the A & P grocery store in their swimsuits. On one hand, Sammy's action is in response to his cynicism about the adult world. This world that he perceives is populated with "houseslaves in pin curlers" with "varicose veins mapping their legs" who drive their shopping carts down the aisle "like sheep." Sammy describes one of the women that he particularly dislikes, calling her a "cash-register watcher." He faults her by remarking, "I know it made her day to trip me up."
On the other hand, the young Sammy is driven by his romantic vision of the teenaged girls who enter the store in their swimsuits. He finds Queenie particularly seductive:
...what got me, the straps were down....there was nothing between the top of the suit and the top of her head except just her, this clean bare plane of the top of her chest down from the shoulder bones....I mean, it was more than pretty.
After he overhears the manager of the A&P, Mr. Lengel, tell the girls that they cannot enter the store because they are not "decently dressed," Sammy impulsively tells his boss that he quits, adding that Lengel should not have embarrassed the girls by speaking to them as he has. Mr. Lengel warns Sammy:
"I don't think you know what you're saying....You'll feel this for the rest of your life."
Thereafter, Sammy impulsively pulls off his apron, punches the "No Sale" button on his cash register, and goes outside where he looks for the girls. However, the girls are gone. It is then that Sammy comments that he "...felt how hard the world was going to be to me hereafter."