The narrator is willing to sacrifice truthfulness, and the monstrous bass he has on the line, because of his youthful crush on Sheila Mant.  The very first line of the story states, "There was a summer in my life when the only creature that seemed lovelier to me than a largemouth bass was Sheila Mant." From the beginning, the reader is aware of how enchanted the narrator is with Sheila Mant.

The narrator asks her to the attend the concert; she surprisingly accepts, and they travel there in the narrator's canoe. Once inside, Sheila states,

“I think fishing’s dumb,” she said, making a face. “I mean, it’s boring and all. Definitely dumb.”

This sets the scene for the final climax of the short story; the narrator loves fishing, and he thinks he loves Sheila Mant.  

Of course, shortly after, the line catches and the narrator finds that a huge bass (judging by its strength on the line) has been inadvertently caught.  He has a choice to make--reel it in and admit his love of fishing, or cut the line to continue his date and possible relationship with Sheila.  After much debate and internal strife, he decides to cut the line.  He sacrifices the truth of his love for fishing and the bass itself because of his enchantment with Sheila; however, he learned an important lesson. At the end of the story, the narrator states,

"There would be other Sheila Mants in my life, other fish, and though I came close once or twice, it was these secret, hidden tuggings in the night that claimed me, and I never made the same mistake again."