The incarnation of a soul is a process of descent into matter in which it forgets what it lived on high. Therefore, arriving on earth, the soul is no longer so aware of the commitments it made vis-à-vis the divine world before descending. But the years pass. As time passes, the soul feels touched by sensations, thoughts and aspirations, at first furtive, then more and more clear. This manifests as a tendency to seek this or that job, or to consider a particular profession. This is what we call a vocation.
And those who have the feeling of being called to perform a task or fulfill a duty, do not hesitate. You may not know explicitly that you have to fulfil a mission you previously accepted; but an unstoppable force ensures that this is the only way you will feel in your element. And whatever the difficulties in your way, you should not change direction.