For every new career, there is usually some sort of seed that is planted.
Some people call it the acting bug.
I got my acting bug after trying out a Meisner class at the suggestion of a friend. Some people have known they wanted to act since they were old enough to entertain their families. For some, acting in their spare time during non-work hours satisfies the bug and allows some creative expression. For others, acting is more than a hobby. No career seems satisfying other than a career as an actor.