Theory:

The poem "The Shed" has several underlying meanings. At a glance, it would seem ordinary. It talks about the speaker's desire to visit a shed in his garden. However, he is overcome by fears and is unable to bring himself to go anywhere near it.

The poem is structured in such a way that his conviction to visit the shed undergoes a transition. As the poem transits from one stanza to another, the speaker's confidence in himself also gets reduced. The refrain at the end of each stanza reflects this change in the speaker's thought process.
 
In the first stanza, he describes how he wants to open the door to the shed; in the second, we see how the speaker's determination had faltered as the boy is satisfied with peeping through the window; in the third stanza, the speaker is evidently scared and wishes that he could, at least, take a peek at the shed from a safer distance. Throughout the poem, the speaker can be seen moving further away from the shed. In the end, the speaker concludes that he would go inside the shed, but it is not going to be anytime soon.

The poem lends a psychological viewpoint of the speaker's mind. Though the speaker’s age is nowhere mentioned, it is fairly understood that he is a child. Hence, the poem also explores the imaginative and cognitive (the ability to think, process, reason, and understand) ability of a child. Though the speaker is scared, he can be seen reasoning with himself (in the fourth stanza) that there is nothing to be afraid of.

While the poem talks about the scary-looking shed in the speaker's garden, it can also mean something more. On a larger scale, the shed can be seen as a metaphor representing the outer world. To the kids, the shed may symbolize the adventure that awaits outside their guarded life.

Several adults are also equally apprehensive about the unknown. The anxiety is so intense that most people follow the saying: 'the known devil is better than the unknown angel'. Doubts, lack of confidence, fears of failure and unwelcome change, and society bring hesitation. The transition and faltering determination of the speaker as the stanzas progress can be compared to the transformation found in people as they age. People tend to become less adventurous as they get older.