If we go to the ten commandments in Exodus 20:2-17 we do not see this commandment of loving one another.Thus Jesus called it a new commandment because it came into being after the ten commandments given in the old testament.Thus it is an additional commandment which came in the new testament being emphasized by Jesus Himself.He emphasized it also by doing .This is because in the synoptic gospel we learn of various situations where Jesus demonstrated love for people.He healed the sick,delivered people from evil spirits,blind received sight etc. In addition he demonstrated great love for us by redeeming us from the law of sin and death.If we love God, Son and the holy spirit we ought to love one another .