In W. D. Wetherell's "The Bass, the River, and Sheila Mant," what causes the narrator to feel ashamed?

In W. D. Wetherell’s short story “The Bass, the River, and Sheila Mant,” we encounter a fourteen-year-old male narrator who, fully inflamed with the onset of puberty, is smitten with the beautiful but aloof seventeen-year-old Sheila Mant.

One of the most obvious (and sometimes debilitating) effects of puberty is a kind of overblown self-consciousness. Suddenly we become very AWARE of ourselves, scrutinizing what we look like, what we sound like, and how we are perceived by others. The narrator, affected by a burning crush on Sheila, wants to approach her, but he lacks confidence, and this makes his feel uncertain when he attempts to make contact:

It was late August by the time I got up the nerve to ask her out. The tortured will-I’s, won’t-I’s, the agonized indecision over what to say, the false starts toward her house and embarrassed retreats—the details of these have been seared from my memory, and the only part I remember clearly is emerging from the woods toward dusk while they were playing softball on their lawn, as bashful and frightened as a unicorn. 

Notice the diction (word choice) Wetherell uses to emphasize the narrator’s diffidence (lack of confidence): nerve, tortured, agonized, indecision, false, embarrassed, retreat, bashful, and frightened.

He also uses a very effective metaphor, comparing himself to a unicorn at the end of the passage. Imagine how you would stare at a unicorn if one suddenly came walking out of the woods—that’s how self-conscious he feels as he walks toward Sheila and her family. He doesn’t feel that he will be accepted or judged worthy of Sheila’s attention.

Part of the narrator’s problem here is that he has chosen to pursue a very difficult objective: Sheila is older, beautiful, and already on the radar of the college boys who row past her while she’s sunbathing. It isn’t likely that he is going to be someone she is particularly interested in. He senses that, but pursues her anyway. He is persistent but not able to avoid the feeling that he will probably fail—hence his feelings of inadequacy and potential failure.