Believers will never be out of the woods in this life when it comes to the possibility of sin...our whole life is nothing but a process of letting go of the old fleshly patterns we have come to rely on and allowing Christ to work in and through us in those very areas. Remember, God is perfecting you until the day of Christ (Philippians 1:6)...He wants you to experience His victory and Life more than you do!
Why do we still sin? Satanic strongholds that must be torn down; fleshly thinking that must be uncovered and replaced with truth; unmet needs that we continue striving to meet in our own strength. There could be many reasons. Although this post is by no means the complete answer, the first step is to understand what happened at salvation and most importantly, to receive personally that God unconditionally loves and accepts you right now apart from any good or bad performance. Won't that just make it worse (i.e. "license to sin")? Actually, no, it's the first step to victory! Take some time to navigate through the site to find some more insights on this very big question!