Each man has a character of his own choosing; it is chance or fate that decides his choice of job.

--Seneca

The context here is Seneca arguing for treating one's slaves well. He observed that one could be a good person and still end up a slave doing a menial job due to circumstances outside your control (like living in a city that got conquered by Rome).

It seems to me that while the external view is largely correct, it can lead to a passive sort of fatalism. The internal view seems better at motivating someone to put in a lot of time and effort, which does give a better chance of being able to exploit chance opportunities.