Most people indeed choose careers they excel at, while others are forced by life circumstances to choose paths completely different from their initial aspirations.
Then there is fixed-mindset thinking. For instance, if generations of ancestors were doctors, their descendants must become doctors too. Since that is what they are most exposed to growing up, they subconsciously feel they should be doctors as well. They know the medical profession inside and out, whereas they have no exposure to or familiarity with other fields; to play it safe, they essentially make the same choices as their parents.
Ran Mouri is a prime example. Her father, Kogoro Mouri, used to be a police officer, and her mother, Eri Kisaki, is a famous lawyer. She could have become a police officer or a lawyer. Moreover, being naturally pretty and well-liked, she would also be suited to being a celebrity or a model. She had two choices: follow her parents into their professions—though Kogoro would likely veto her becoming a police officer, he might accept her following in Eri's footsteps as a lawyer—or carve out a path of her own by becoming a celebrity or a model. As for becoming a detective... that is hard to judge, especially with two prime examples sitting right before her eyes.
Chapter 71