Can passion or emotion increase the moral goodness of human action, or does it always decrease it? It might seem at first that it always decreases it, since human or moral action is an action that is chosen deliberately, which implies a rational judgment. But passion tends to win reason's judgment over to itself: whenever we feel as though something is pleasant or unpleasant, good or bad, possible or impossible, etc., we will be inclined to judge the matter to be really so. Insofar as passion tends to sway reason over to itself, it impedes the judgment of reason. Thus it seems that passions or feelings make our judgments, and in consequence our decisions, less true, and consequently less good from a moral perspective.
Now, given that feelings or passions can be in accord with reason and with man's true good, and that they can be indirectly voluntary, being willed or permitted (see the previous post The Moral Value of the Passions), it is clear that this is something good. It is a moral good not only to do an act of kindness for someone, but it is also a moral good to feel kindly toward them.
Yet it is not simply a matter of two separate moral goods. Feeling kindly, if one also wills to be kindly, may increase the person's overall attention to and intention of being kind, and in this way intensify and perfect the human act as a voluntary act.
What about the objection, that passion always hinder rational judgment and choice? The first response to this is that sometimes passions or feelings actually come from rational judgment and choice, and so if they affect reason and choice afterward, it is by way of supporting them, not tearing them down. In such a case the passion does not decrease the moral goodness of an action. We may and should admit, however, that a judgment and choice which is largely the result of passion or emotion is less good or less bad than a similar judgment and choice that does not result from passion. Thus someone who strikes another person out of anger performs an act less morally bad than someone who strikes another person out of simple dislike for that person. Similarly someone who does an act of kindness for another person simply because of feeling pity for them, performs an act less morally good than someone who does an act of kindness for another person because that other person is a fellow human being and created in the image of God.
Does this mean, though, that the ideal is to never have any passions or emotions except those that are the result of rational judgment and choice? Yes and no. Considering only the abstract relationship between passion and reason, this is an ideal. But taking into account our actual situation and our limited ability to make rational judgments without the data of emotion, such an absolute priority of reason over emotion is not an ideal to be striven for. First of all, it is impossible. Secondly, it is actually undesirable for us in our present state, for several reasons: (1) we often need to react somehow or other to an event, before we have time to think. And it is better to react well in most such cases, and badly in a few, than to always do badly by failing to react in time; (2) we would often overlook many important aspects of a situation or of a person, if various feelings did not direct our attention to them; the feeling of pity, for example, often draws us to give more and more appropriate attention to the need of other persons than we would otherwise.