a comment explaining my disagreement or disapproval is more productive

Also more costly and sometimes useless, if somebody else has already explained the same reasons.

doesn't make the downvotee feel bad

Disagreement often makes people feel bad, even if explained (and sometimes more because it is explained).

and I frequently get karma because people will upvote my comment

If all people never downvoted like you suggest, having positive karma would mean nothing. Paradoxically, the worth of karma you get for not downvoting depends on other users' occasional downvoting.