What is the theme of the poem "My heart leaps up " by Wordsworth ?

The idea of Wordsworth's, "My Heart Leaps Up," is that life isn't worth living if one does not have an intimate relationship with nature. 

The speaker's heart metaphorically leaps up when it sees a rainbow in the speaker's present, as it did when he was a child, and as it will in the future when he will be old--if it doesn't when he is old, he might as well die. 

And poetry is not that subjective.  An intelligent interpretation of a poem must stem from evidence within the poem.  There's nothing in this poem about paintings or loved ones or about growing old well.  The poem's about having an intimate relationship with nature every day of one's life, whether one is a child or a man.