The things we love tell us what we are. — Attr. St. Thomas Aquinas
This one is easy. It comes from Peter Kreeft’s Socrates Meets Kant, p. 183, which cites the Summa Theologica I-II, 34, a4. The portion of that passage which Dr. Kreeft seems to have in mind is this:
And therefore man is reckoned to be good or bad chiefly according to the pleasure of the human will; since that man is good and virtuous, who takes pleasure in the works of virtue; and that man evil, who takes pleasure in evil works.
As you can see, Dr. Kreeft paraphrased St. Thomas, which is perfectly acceptable. It’s just that people picked up his paraphrase as if it were a quotation, which it is not.
Update 2/15/16: A commenter below notes that the exact quotation can be found on p. 22 of Thomas Merton’s Thoughts in Solitude. I don’t own a copy and the previews on Google Books don’t show this page, but I am confident that the comment is correct. Dr. Kreeft must have confused his Thomases.