Probably one of a few reasons:
1. It's an old poem, or possibly it was translated. In the German language, for instance, all nouns are capitalized, instead of only proper ones. So if a poem was translated from German, the translator might just decide to leave them that way. Old English seems to have often been the same -- the grammatical rules were just different back then.
2. A typo. Obviously this doesn't account for very many of the capitalizations, but perhaps occasionally.
3. The most likely answer. It does have significance, but perhaps not an obvious one. Perhaps the poet meant it as a form of personification, by changing the word's status from that of a common to a proper noun? Of course, this doesn't work for adjectives, but there are other ways that this could be taken. Likely, the word had some sort of exceptional personal impact or importance in the mind of the poet, and he or she, knowing that perhaps it wouldn't strike everyone in the same manner, wished to convey this feeling in a relatively flowing and unobtrusive way.