What does varying your sentence length do? A.
Makes your writing more interesting

B.
Allow for better “flow”

C.
Both A and B

He feels that he way because

A. they give the reader hope. if the whole book was dark & unhopeful it’d be a little less interesting, little moments make it more interesting to read because then you want to know what happens. if it’s all dark there’s no point in reading if you already know the outcome of it.