user223: This has been mentioned by users in various questions, and having personally seen it to my question(s), I wonder whether it might be a good idea to make posting a comment explaining a downvote a requirement to down vote a question or answer. This makes it easier to understand the reasoning behind the vote.
That said, by the same logic, it ought to be needed for a up vote too. However, an upvote is understood to mean good quality of question by default. So, my question is specific to downvotes.

user69: This is a change that would have to affect the entire Stack Exchange Network.  For that reason, it would be discussed on the network meta, Meta Stack Overflow.  
Indeed, this has been discussed in great detail and proposed many times.  In summary the community disliked the concept of the resulting loss of vote anonymity (leading to a small risk of revenge downvoting) and (particularly on Stack Overflow) the extra effort required to write a comment on unsalvageable questions.  
The central discussion is located here, ending when Jeff added a pop-up for users with <2k Rep (though I'm sure I still see it myself?):

Forcing a comment will end in even worse results. Jeff Atwood

If you feel strongly and have new arguments I'd encourage you to voice your opinion on MSO where you will get a larger audience.  

223: <p>This has been mentioned by users in various questions, and having personally seen it to my question(s), I wonder whether it might be a good idea to make posting a comment explaining a downvote a requirement to down vote a question or answer. This makes it easier to understand the reasoning behind the vote.</p>

<p>That said, by the same logic, it ought to be needed for a up vote too. However, an upvote is understood to mean good quality of question by default. So, my question is specific to downvotes.</p>
 69: <p>This is a change that would have to affect the entire <a href="http://stackexchange.com/sites/">Stack Exchange Network</a>.  For that reason, it would be discussed on the network meta, Meta Stack Overflow.  </p>

<p>Indeed, this has been discussed in great detail and proposed many times.  In summary the community disliked the concept of the resulting loss of vote anonymity (leading to a small risk of revenge downvoting) and (particularly on Stack Overflow) the extra effort required to write a comment on <em>unsalvageable</em> questions.  </p>

<p>The central discussion is located <a href="https://meta.stackexchange.com/questions/135/encouraging-people-to-explain-down-votes">here</a>, ending when Jeff added a pop-up for users with &lt;2k Rep (though I'm sure I still see it myself?):</p>

<blockquote>
  <p>Forcing a comment will end in even worse results. <strong>Jeff Atwood</strong></p>
</blockquote>

<p>If you feel strongly and have new arguments I'd encourage you to voice your opinion on MSO where you will get a larger audience.  </p>
