Romans 3:23

23 for all have sinned and fall short of the glory of God,

Why does God allow us to sin? I am not sure, but think about this. One day we will be united with Jesus in heaven and according to the Bible we will be perfect just like Him. In that era, we will be able to praise Him for what He has done for us, because we will remember who we were apart from Him and apart from His sacrifice for us. If we had not been subjected to sin, seen the horrors of it and struggled with it for a season, we would not be able to appreciate fully what He has done, we could only be told about it. We would only know that we were perfect, just as He is perfect and would never know experientially who we would be without Him.