an upvote is similar to a Like on Web2 and a downvote is like Dislike which some Web2 has. Twitter calls it a downvote. So, the principle of being able to express both like and dislike is not new, it's just more noticed because both actions come attached to rewards.
As Dreemsteem wrote in her post, those rewards are not ours until the post pays out. So an upvote is potential rewards and a downvote reduces the potential.
Feeling disappointment at receiving one for any reason, right or wrong, is normal. Keeping it in perspective is important.