You won't be punished but as cog says you won't be rewarded either and it might not count towards your constructive reviews. Try reading your favourite piece of fiction and analyse it in more depth, see what you like about it. That should give you some experience.
The only way I know of to get past the "I don't know why" is to force yourself to answer it. That's why giving constructive critiques are much more difficult than just giving a review.
