It's best to come to any kind of forum where there's a diverse crowd with a thicker skin.
Here is a short, non-exhaustive list of reasons why you'll get downvoted:
* Someone accidentally clicked it; there's no undo.
* You said something factually incorrect.
* You said something emotionally horrible or embodying prejudice.
* You said something that didn't contribute to the discussion in a meaningful way. One word comments are a prime example of this.
* Similar to the above would, your comment is profoundly off-topic; bible verses are an example.
* You said something that was poorly reasoned.
* You're trolling, or pissing people off beyond reason without merit.
* You're promoting in an unpalatable way.
There is a mechanism for explaining the downvote: a reply to the downvoted comment. Most of the time someone will explain why the downvoting happened, if it's not already painfully obvious, or it's a troll comment.
At the end of the day, it's only an integer in a database and I don't support this idea. Also, there's a flag button for pretty egregious comments and posts, which would make downvoting for the purposes of flagging redundant.