Thomas argues that it has to be (ST IIa IIae, 22, 2). By “created,” he means a habit or power which the soul can exercise freely. Here he opposes Peter Lombard (“Magister”), who had argued that the love by which we love our neighbor is in fact the same love by which the Father loves the Son — namely, the Holy Spirit, the “vinculum caritatis,” indwelling each believer. Thomas insists that this cannot be, because that would mean that to say, “John loves Mary,” would really be equivalent to saying, “The HS loves Mary.” But in that case, “John’s” love earns him no merit, and so is not salvific, because he did not love freely; it really isn’t his love at all.
This reasoning seems thin to me — Thomas has no problem elsewhere conceptualizing how God (the Father’s) providence can contain our own acts of free will without coming into conflict with them. Perhaps what’s really at work here is a deficient pneumatology — if we conceive of the Holy Spirit simply as an “energy” of God at work in the soul, then yes, its operations would indeed preclude our free choosing. But if he is true God of true God, then the Spirit’s willing that we will freely is not a contradictory act.