Objection 1. It would seem that doubts should not be interpreted for the best. Because we should judge from what happens for the most part. But it happens for the most part that evil is done, since "the number of fools is infinite" (Ecclesiastes 1:15), "for the imagination and thought of man's heart are prone to evil from his youth" (Genesis 8:21). Therefore doubts should be interpreted for the worst rather than for the best.
Objection 2. Further, Augustine says (De Doctr. Christ. i, 27) that "he leads a godly and just life who is sound in his estimate of things, and turns neither to this side nor to that." Now he who interprets a doubtful point for the best, turns to one side. Therefore this should not be done.
Objection 3. Further, man should love his neighbor as himself. Now with regard to himself, a man should interpret doubtful matters for the worst, according to Job 9:28, "I feared all my works." Therefore it seems that doubtful matters affecting one's neighbor should be interpreted for the worst.
On the contrary, A gloss on Romans 14:3, "He that eateth not, let him not judge him that eateth," says: "Doubts should be interpreted in the best sense."
I answer that, As stated above (3, ad 2), things from the very fact that a man thinks ill of another without sufficient cause, he injures and despises him. Now no man ought to despise or in any way injure another man without urgent cause: and, consequently, unless we have evident indications of a person's wickedness, we ought to deem him good, by interpreting for the best whatever is doubtful about him.
Reply to Objection 1. He who interprets doubtful matters for the best, may happen to be deceived more often than not; yet it is better to err frequently through thinking well of a wicked man, than to err less frequently through having an evil opinion of a good man, because in the latter case an injury is inflicted, but not in the former.
Reply to Objection 2. It is one thing to judge of things and another to judge of men. For when we judge of things, there is no question of the good or evil of the thing about which we are judging, since it will take no harm no matter what kind of judgment we form about it; but there is question of the good of the person who judges, if he judge truly, and of his evil if he judge falsely because "the true is the good of the intellect, and the false is its evil," as stated in Ethic. vi, 2, wherefore everyone should strive to make his judgment accord with things as they are. On the other hand when we judge of men, the good and evil in our judgment is considered chiefly on the part of the person about whom judgment is being formed; for he is deemed worthy of honor from the very fact that he is judged to be good, and deserving of contempt if he is judged to be evil. For this reason we ought, in this kind of judgment, to aim at judging a man good, unless there is evident proof of the contrary. And though we may judge falsely, our judgment in thinking well of another pertains to our good feeling and not to the evil of the intellect, even as neither does it pertain to the intellect's perfection to know the truth of contingent singulars in themselves.
Reply to Objection 3. One may interpret something for the worst or for the best in two ways. First, by a kind of supposition; and thus, when we have to apply a remedy to some evil, whether our own or another's, in order for the remedy to be applied with greater certainty of a cure, it is expedient to take the worst for granted, since if a remedy be efficacious against a worse evil, much more is it efficacious against a lesser evil. Secondly we may interpret something for the best or for the worst, by deciding or determining, and in this case when judging of things we should try to interpret each thing according as it is, and when judging of persons, to interpret things for the best as stated above.
Source: Summa Theologica (New Advent)