Why is the Sacrament of Holy Orders considered a Sacrament of Vocation?

Holy Orders is considered a sacrament involving a vocation since one must be "called" to receive it and not everyone is. The calling or vocation involves the desire to pursue holy orders as well as the Church's invitation to receive them. Since Holy Orders dictates an entire way of life, that is, that once received you are expected to live the life of a religious in the service of God, the sacrament is life-altering event, a vocation.