"if everything happens that can't be done" takes place in a world in which everything is possible, so of course there's going to be a good amount of awe and amazement. We see this in the poem's magical transformations, its hyperbole, and in its pure glee. Love is awesome, and awe-worthy in this one, so prepare yourself for some gasps, swoons, and sighs.
The awe in this poem is caused by the speaker's love, which makes even the most mundane things, like leaves, amazing.
The speaker finds more awe and amazement in the natural world than he does in books because he's a fan of visual beauty.