via In the Beginning…’: A Catholic Understanding of the Story of Creation and the Fall:

Human beings are relational, and they possess their lives – themselves – only by way of relationship. I alone am not myself, but only in and with you am I myself. To be truly a human being means to be related in love, to be of and for. But sin means the damaging or the destruction of relationality. Sin is a rejection of relationality because it wants to make the human being a god. Sin is loss of relationship, disturbance of relationship, and therefore it is not restricted to the individual. When I destroy a relationship, then this event – sin – touches the other person involved in the relationship. Consequently sin is always an offense that touches others, that alters the world and damages it.