My heart leaps up when I behold A rainbow in the sky: (1-2)
The poem starts by telling us something about the speaker: he gets really super-happy whenever he sees a rainbow. Right away we know that he finds this part of his identity very important. (We doubt our speaker would start a poem off with something unimportant, like the fact that he likes to brush his teeth.) Throughout the poem, we keep in mind that the person we're reading about really loves nature.
So was it when my life began; So is it now I am a man; (3-4)
The speaker is walking us through his identity from birth to adulthood here. That's a long walk in a short poem. He got excited about rainbows as a child, and he still does as a man. So we know that his personality is fairly constant, at least in terms of his love of nature.
So be it when I shall grow old, Or let me die! (5-6)
The speaker considers rejoicing in nature so integral to his identity that, if he stopped loving nature, he would want to die. That's intense. We've now followed the speaker through his entire lifespan, or at least the lifespan he projects for himself.