The speaker loves nature a bunch. Like, a whole bunch. He loves it so much, in fact, that he expresses a wish to die if he is no longer thrilled at the sight of a rainbow. While he's still alive, he wants nature in his life, every day. It's possible that he even sees nature as a form of religion or spirituality.
Lines 1-2: These lines introduce the joy the speaker feels at seeing a rainbow. The first line is an example of personification: hearts don't leap, people do. This personification gives us an image of the heart's jubilation at the sight of the rainbow.
Lines 3-6: These lines address human nature and the slow changes from birth to death. Here, the natural world acts as a constant amid the change of aging. No matter how old our speaker gets, how far in time he moves from his childhood, he'll always carry the joy of nature with him. See more about this under "Age" in our "Symbols, Imagery, and Wordplay" section.
Lines 8-9: The idea of nature as a form of spirituality or religion is introduced in these lines. Nature is more than just our undeveloped surroundings now. It's a source of spiritual fulfillment for our speaker.