We can see that there are four stanzas: two quatrains and two tercets. Each verse has ten syllables, so we conclude that it is a Petrarchan sonnet with the following full-rhyme:
The poet suggests that there is a connection in nature (winter-summer). He thinks that we can find poetry in nature. He feels that nature transmit him poetry, but it is necessary to have a “romantic soul” to feel nature. In few words, Keats is talking about harmony : his soul is connected with nature, and this never ends.
Nature is seen as something cosmic, and because of this I can say that he is pointing out to the sublime.
We can see that in Romanticism the connection and harmony was based on soul.
Nowadays I think that it is quite difficult to feel connected with nature. What do you think?