It hurts to get stronger. Have you ever lifted weights or trained for some sport or athletic competition?
We don't tend to like pain. So we often go the path of least resistance. And we suffer for it. In the long run, it's actually more painful (to you and others) to stop growing and choose atrophy.
Don't let pain put you off. The Bible makes it abundantly clear: the gain is worth the pain.
1 Timothy 4:7-8 (emphasis added)
...train yourself for godliness; for while bodily training is of some value, godliness is of value in every way, as it holds promise for the present life and also for the life to come.1 Corinthians 9:24-27 (emphasis added)
Do you not know that in a race all the runners run, but only one receives the prize? So run that you may obtain it. Every athlete exercises self-control in all things. They do it to receive a perishable wreath, but we an imperishable. So I do not run aimlessly; I do not box as one beating the air. But I discipline my body and keep it under control, lest after preaching to others I myself should be disqualified.Hebrews 12:3-14 (emphasis added)
Therefore...let us also lay aside every weight, and sin which clings so closely, and let us run with endurance the race that is set before us, looking to Jesus, the founder and perfecter of our faith, who for the joy that was set before him endured the cross, despising the shame, and is seated at the right hand of the throne of God.
Consider him who endured from sinners such hostility against himself, so that you may not grow weary or fainthearted. In your struggle against sin you have not yet resisted to the point of shedding your blood.
And have you forgotten the exhortation that addresses you as sons? "My son, do not regard lightly the discipline of the Lord, nor be weary when reproved by him. For the Lord disciplines the one he loves, and chastises every son whom he receives." It is for discipline that you have to endure. God is treating you as sons. ...he 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 to those who have been 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 put out of joint but rather be healed. Strive for peace with everyone, and for the holiness without which no one will see the Lord.Does your life look like the gain (of godliness and the imperishable wreath and sharing in God's holiness and the yield of the peaceful fruit of righteousness and seeing the Lord) is worth the pain? It will really hurt if you let the pain put you off.