My deeds aren’t things without any relation to who and what I am; they define me (for better or for worse). So says JPII in VS §71:
Human acts are moral acts because they express and determine the goodness or evil of the individual who performs them. They do not produce a change merely in the state of affairs outside of man but, to the extent that they are deliberate choices, they give moral definition to the very person who performs them, determining his profound spiritual traits. This was perceptively noted by Saint Gregory of Nyssa: “All things subject to change and to becoming never remain constant, but continually pass from one state to another, for better or worse… Now, human life is always subject to change; it needs to be born ever anew… But here birth does not come about by a foreign intervention, as is the case with bodily beings…; it is the result of a free choice. Thus we are in a certain way our own parents, creating ourselves as we will, by our decisions”. [emphasis in original]
This is why the Church says that virtues and vices are “second nature” to us. They aren’t really a part of what it is that makes us human, but they do in a sense define us. Consequently it is imperative that we not dally with sin; it is critical that we take holy living seriously.