A few more verses you might like that explain why we have to strengthen ourselves as soldiers, and continue to fight the good fight of faith:
This charge I commit to you, son Timothy, according to the prophecies previously made concerning you, that by them you may wage a good warfare... (1 Timothy 1:18)
1 You therefore, my son, be strong in the grace that is in Christ Jesus. 2 And the things that you have heard from me among many witnesses, commit these to faithful men who will be able to teach others also. 3 You therefore must endure hardship as a good soldier of Jesus Christ. 4 No one engaged in warfare entangles himself with the affairs of this life, that he may please him who enlisted him as a soldier... I endure all things for the sake of the elect, that they also may obtain the salvation which is in Christ Jesus with eternal glory. 11 This is a faithful saying: For if we died with Him, we shall also live with Him. 12 If we endure, we shall also reign with Him. If we deny Him, He also will deny us. 13 If we are faithless, He remains faithful; He cannot deny Himself. (2 Timothy 2:1-13)
If you are a threat to the enemy, he directs his attacks at you especially, and after a time it can really beat you down emotionally and spiritually. Hence the need to be reminded of the fact that we are called of God and have a job to do, and that we should keep stirring up the gifts that are in us because we are in a war, and our gifts serve as weapons in our hands against the tactics of the kingdom of darkness.