Wov. Twoo wov. The speaker of "My Heart Leaps Up When I Behold" loves nature so much that he would rather die than lose the capacity to feel joy at the sight of a rainbow. That's some serious love right there. He's loved nature since he was born and wants to keeping loving it until he dies. He views nature as a way to tie together every day of his life in peace and in joy.
He's got to have it. The speaker is dependent on the daily presence of nature in his life, or else his life will lose all meaning.
Meh. Even though the speaker desires the natural world in his life, his memory of the importance of nature to him as a child is enough to sustain him. He's exaggerating when he says "let me die!"