How does the down-vote thing work?

I've joined reddit less than a month ago, and though during this time I've learned some things, I still don't understand others.

(Brief aside: which is the difference between a reddit downvote and what we know as a dislike?). This first point links to my real question: which is the criteria for downvoting someone? Because see:

for example, in a subreddit somebody asked me if I had read "x" book. As I hadn't, my answer was like "no, I haven't read that book, blah blah". Right after that, I got like two downvotes in the comment. I mean, what did I do wrong? Am I guilty for not having read a random book? (Before you say something, both the question and the answer were related to the topic).

Another example is when someone downvoted me just because I'm a non-language speaker and sometimes, accidentally of course, I make spelling mistakes. My reaction was like ok dude, I'm not Queen Elizabeth grandaughter (no offense), English is not even my first language.

This happened to me multiple times. I didn't say anything wrong or offensive(? and I received a bunch of downvotes.

I'm not complaining, I just want to understand why. Do you know if there's a criteria to be followed, or some people are just assholes? Thanks!