My heart leaps up when I behold A rainbow in the sky: (1-2)
These lines just make us happy. We can picture a man looking up at the sky, a huge smile on his face, pausing to revel in the beauty of nature. Rainbows are just cool—it's as if nature were apologizing for raining on us by giving us a show afterwards. And who doesn't smile at least a little when they see one?
So be it when I shall grow old, Or let me die! (5-6)
Now our speaker gets a little more intense. He goes from talking about how his heart leaps up when he sees a rainbow to talking about how he'd want to die if he ever lost that feeling. We at Shmoop think this may be a little extreme, but we agree that the inability to appreciate a rainbow would be a pretty sad thing.
The Child is father of the Man; (7)
Not all children are innocent or happy, we must admit. Still, we tend to associate these traits with them. Children, if they are lucky, are protected from the troubles and worries of the world. They are free, in their innocence, to take pleasure in the little things in life—unlike many adults, who no longer notice or have time for them. In this line our speaker is telling us that as men (or, in general, adults) we are all derived from children. The speaker is suggesting that we should hold on to our childlike enthusiasm for nature for our entire lives.