You can ask someone why they chose to do something with the phrase "What made you ___?"
For example, in a job interview, the interviewer might ask this:
A: What made you decide to apply for this position?
B: Well, I've always wanted to work here, so when I saw that you had an open position, I jumped at the chance.
"What made you" has a softer sound than "Why" which might seem a little rough.