I am wondering why do my reputation points go down if I downvote someone else's post?
Thank you for your time.
This happens because your down vote will remove 2 rep from that user. You lose 1 rep as well to prevent you from gaming the system. We couldn't let you go around taking rep away from others without making you consider if it's really worth it, could we?
If you can't justify the small loss to express your opinion on the particular post, then there's no need to down vote. If other people feel more strongly than you, they'll take care of it.