If someone down-votes, just assume it is for one of these three reasons:
- They feel that "This question does not show any research effort; it is unclear or not useful". This is the text on the ugly newly-designed circled downward-facing triangle button someone clicked. The text is the same for all StackExchange sites.
- They feel like the OP did not follow the good advice in the site-specific help and in the quick start help provided for new contributors (or did not bother to read it).
- They did not like the question for another reason. Maybe they were hoping that it would get sufficient down votes to get off the front page of the site.
If you get down votes, check if you can address 1. or 2. and edit your question accordingly. If there is nothing you can do to improve the question, accept the reality that some people read it and felt sufficiently strongly about it that they decided to spend extra time to click the down button.
Another thought (not mentioned by the OP, but this is meta):
Take a look at the views and at the vote tally (and if you have sufficient reputation, the number of up and down votes). Here is a random example:
So 178 users saw this question, and 2 answered (they must have thought the question was worth their time). There were 178 chances to get an up vote. When I looked, there were 3 up votes and 4 down votes, and I did not vote, even though I wrote an answer (I try to remember to not answer if I don't like the question, and to up-vote if I like the question and write an answer).
So when you post a better question (or update it and with that push it to the top of the question feed), you have to chance to convince the e.g. 171 non-voters to chime in. This is a better goal than trying to understand why individuals down-vote. Even if you knew their reason, chances are low that you could change their mind, and chances are high you would get into an argument about it.
If you are still reading and thinking about voting, here is a plug to vote more often. If you do, votes will better reflect the entire community's evaluation of questions and answers. For this reason, SE changed the appearance of the buttons (to entice us to click, not to have a beautiful layout).