The poem doesn't actually have a title (thanks a bunch, Wordsworth), so we use the first line: "My Heart Leaps Up When I Behold." That might explain why the title seems a little long, or somehow incomplete.
Yet this first line-title works pretty well as a title in a way, since it keeps the object of joy open. Of course, as we read the poem, we find out that it's a rainbow that makes the speaker's heart do jumping jacks. When we just read the title, though, it could be anything that has that effect on him—seeing a favorite person or animal, a kiss, or even winning the lottery, for all we know. Though this line is not technically the title, it's important in setting up the rest of the poem. The first six lines all directly refer back to it.
And, while the last three lines are not technically, grammatically connected to the first line, they are also colored by it, too. When we hear the speaker talking about "natural piety," we guess that this "natural piety" explains why his heart is leaping up at the sight of the rainbow (as opposed to some weird medical condition).