Human nature is inherently weak to do what it truly wants and live righteously.

(Rom.7.21-25So I find this law at work: Although I want to do good, evil is right there with me. For in my inner being I delight in God’s law; but I see another law at work in me, waging war against the law of my mind and making me a prisoner of the law of sin at work within me. What a wretched man I am! Who will rescue me from this body that is subject to death? Thanks be to God, who delivers me through Jesus Christ our Lord! So then, I myself in my mind am a slave to God’s law, but in my sinful nature a slave to the law of sin.)

Evidence? Just try keeping a new year resolution long enough. Or discussing with someone hooked on a bad habit.
This stems from the fall of the first man; Adam, from the place of close relationship with God after being deceived into sin by satan.
God still longed for a relationship with man and to achieve this He had to "Help" mankind gain righteousness by sending His Only Begotten Son Jesus to pay the price for our sins.
God is righteous and for Him to get something, He had to follow the rules which He Himself laid down that nothing valuable goes for nothing. Hence the need for the sacrificial death of Jesus on the Cross of Calvary.
Man could not save himself because we were already tainted with sin. Only the sacrifice of a Righteous One could be acceptable in God's presence as a reasonable atonement for our sin.
Therefore by accepting God's free gift of righteousness, that inherent weakness in us disappears making it possible for us to live Godly lives.
If this is so, why do some Christians still struggle with sin?
1. Some haven't realised the value of what God did for us through Christ' sacrifice on the Cross. They haven't realised they now have the inherent power from God to live above sin, sickness, poverty and death. They still struggle to achieve these things by their limited human abilities. This is what most religious people do; trying to achieve freedom from sin and death through numerous works and activities like festivities, animal sacrifices and constant religious gatherings and exercises.

2. They haven't also started walking fully in the Power of the Holy Spirit. Although we have been saved, the devil will still make numerous attempts to pull us back into worldly activities and we can only win this battle when we walk according to the Spirit and in the full armour of God. Victory becomes ours as we do so and manifesting God's glory and our freedom from sin becomes possible.

Accept Jesus today.