The thing is, reasons Y and Z for me very often are "because I want to". I don't even really want to fluff it up and call it something nauseating like "professional growth". I just followed my curiosity. My personal projects are a safe space from stiff professional judgement (or at least were).
You're my favourite kind of employee/colleague. Someone who knows something about what they're doing because they took it apart and put it back together.