Roman's 7:21
Roman's 7:21: "So I find it to be a law that when I want to do right, evil lies close at hand." (ESV)
Paul is now on the topic of sin living in our flesh at this point in Romans. Earlier he stated that "For I know that nothing good dwells in me, that is, in my flesh." (v 19). There he says that because he is a man that was born into this world, he is already going to sin in his life. Sin is of the flesh, the bodily desires of a man. God is the Spirit in us, our spiritual desires not of this earth like our flesh, or, sin. When we want to do what is good and right in God's eyes, that's going to upset Satan, a lot, which is why he is right there at the very moment we want to do something right. He's wanting and trying to make us slip up and do what is displeasing to our Father.
So the lesson: doing what is good isn't always going to be easy, we will go through turmoil and constantly fight our flesh to do what is not of the flesh. There's a spiritual battle in each of us that goes on whenever we make decisions. It's our goal and our job to make sure that we don't let the temptations of Satan and our flesh get the best of us in those moments. Choosing to do good will not always be bad, but it will always be good and rewarding.