What is the Catholic explanation for temptation?

What is Temptation?

(Latin tentare, to try or test).

Temptation is here taken to be an incitement to sin whether by persuasion or by the offer of some good or pleasure. It may be merely external, as was the case of Christ's encounter in the desert after the forty days' fast; or it may be internal as well, inasmuch as there is a real assault upon a person's will power. It arises sometimes from the propensity to evil inherent in us as a result of original sin. Sometimes it is directly chargeable to the intervention of the Devil, who can furnish the imagination with its sinful subject-matter and stir up the lower powers of the soul. Not infrequently both causes are at work. Temptation is not in itself sin. No matter how vivid the unholy image may be, no matter how strong the inclination to transgress the law, no matter how vehement the sensation of unlawful satisfaction, as long as there is no consent of the will, there is no sin. [...] - Temptation | New Advent.

Since temptation is [...] taken to be an incitement to sin it cannot be from God for

for God cannot be tempted with evil and he himself tempts no one.

The above is from the Epistle of St. James. Verses 1:12-18 Trial and Temptation open as follows:

Blessed is the man who endures trial, for when he has stood the test he will receive the crown of life which God has promised to those who love him.

Therefore while we pray as Jesus taught us:

And lead us not into temptation, but deliver us from evil.

we know it is necessary that temptations come and that we are to resist them [cf. 1 Pt 5:9] be they from the devil, world, or our flesh, because if we do so to the end, we will have stood the test and we will receive the crown of life which God has promised to those who love him [cf. Jm 1:12]

Good question. As it turns out, there isn't an official Catholic position. That is, the Catechism does not address the question of why temptation occurs. What follows is an argument combining the thinking of several different Catholic scholars.

First, the definition of temptation is

An attraction, either from outside oneself or from within, to act contrary to right reason and the commandments of God (Glossary to the Catechism)

To never be tempted would mean that God would have to both control your thoughts (making anything wrong repulsive to prevent internal temptation) and the actions of other people (to prevent external temptation). This would clearly interfere with free will. As Charles Pope says:

temptation is the "necessary" result of freedom. As free persons who are invited to love God and to say yes to his will, we must be permitted to say no. There must be real alternatives to what God offers. If God could force our yes, then we would not be free and our yes would have no real meaning.

Additionally, facing temptation is actually beneficial to the believer. As Father Fortea says:

Without temptation, there would be no opportunity for us to grow in virtue every time we successfully resist the allure of sin and remain faithful to God’s will... In other words, we could say that God permits a certain amount of darkness in this world (temptation and sin) so that a purer and brighter light (virtue and salvation) can be gained. So God allows temptation because He knows that much good can come from it if we resist.

On this point, see also James 1:2-3:

My brothers, consider it a great joy when trials of many kinds come upon you, for you well know that the testing of your faith produces perseverance (NJB)

God will, however, prevent us from facing temptations beyond our ability to resist. (If we truly could not resist, we would not be culpable for our actions.) See 1 Corinthians 10:13:

None of the trials which have come upon you is more than a human being can stand. You can trust that God will not let you be put to the test beyond your strength, but with any trial will also provide a way out by enabling you to put up with it.

The Catholic Harbor of Faith and Morals takes this "moral shaping" idea a step further, writing:

God allows us to be tempted out of mercy, for the good of our souls... He allows us to be tried by temptation to give us the opportunity of manifesting our loyalty to Him, and acquiring a claim to the recompense He promises us. Thus He has only our welfare in view. (my bold)

On this point, the Vulgate version of Tobit is cited:

And because thou wast acceptable to God, it was necessary that temptation should prove thee. (12:13, Douay-Rheims)

Additional that site offers the following positive benefits of facing temptation:

• Rouses the believer from an otherwise apathetic state
• Cleanses imperfections (by making us aware of them)
• Humbles us
• Increase spiritual strength (you can't grow in faith by standing still)
• Make us more charitable in our dealing with others

It concludes:

Thus we see that the tempter does us good service, and his temptations are steps in the ladder which leads to heaven. Therefore let him who is tempted rather pray for strength to resist the temptation than for its entire removal.