Happy, happy! Joy, joy! From the start, we learn that the speaker's heart leaps up when he sees a rainbow. This enthusiasm, we find out, runs throughout his entire life and is largely created by his relationship with nature and with childhood. Think about the things that make your heart leap with happiness when you consider the theme of happiness in "My Heart Leaps Up When I Behold."
The speaker in this poem can only draw happiness from the beauty of nature. Like Sinéad O'Connor sang, nothing compares to nature for our speaker.
The speaker of this poem feels that, if one of the main sources of happiness in his life no longer pleased him, it would not be worth living. Bummer.