Many times, doctors and physical therapists have to allow pain to be caused in order to heal you.  Think of a doctor who reconstructs your knee, and the therapist who starts to get you to move it ~ very painful!  But if they didn't do that, you'd never walk again.

God, our spiritual physician, works in the same way.  Why does God allow temptations?  Why does God allow trials and suffering and failures?  Why does he let the devil torment us?  Because the muscle of our weakened will needs exercise and strengthening!

"God treats you as sons, for what son is there whose father does not discipline them?  Our earthly fathers discipline us for a short time, as it seemed best to them, but God the Father disciplines us for our good, that we may share his holiness.  For the moment, all discipline seems painful rather than pleasant, but later it yields the peaceful fruit of righteousness for those who are trained by it.  Therefore, lift your drooping hands and strengthen your weak knees, and make straight paths for your feet, so that what is lame may not be disjointed, but rather be healed."

What needs healing in us?  First of all, our pride.  God allows us to fail, to fall, to suffer trials and temptations so that we realize we don't have it all together, we can't do it on our own, we don't have the strength.  He allows us to experience what it's like not to be upheld by His power and strength so that when we are being upheld and defended by Him, we'll know it's not from us, but by His grace!

Our weakened will also needs healing, and like any muscle, it needs exercise to get strong!  And, just like our bodily muscles are only exercised when there's an opposition pushing against them, so too the only way our weak will is going to get exercise is if there's an opposition to what we know by reason and God's revelation is true and good and excellent.  This of course, is our fallen passions and appetites, which the demons try to exploit and inflame, to get us to choose against our reason, expressed in God's commandments.  How many times do we know what's good and right and just, yet find ourselves powerless to do it because of the force of bad habit or all kinds of forces pulling on our will to remain calcified ...

The opposition of temptations, trials, suffering, though often painful, can be turned to good when we see in them the opportunity to exercise our will and grow strong.  The hardest moments are at the beginning, when we choose to break an addiction or bad habit, or start a new good habit.  The grip of our fallen passions is strongest then, but over time, one choice at a time, we can grow stronger, until our desires actually become trained to love and do what is good and right with ease, just as with more exercise comes a desire for it.

We in no way can do this alone.  To win the battle, we need the Holy Spirit, God's power!  We need the power that flows from the God-man, Jesus Christ, available to us in the seven Sacraments, especially Penance and the Eucharist!

So, if you're being tempted or are in the midst of a trial, rejoice! because God's treating you as His spiritual son or daughter, an athlete-in-training, to strengthen you to run the race set before you and become strong in your will for holiness!