this is such a broad and possibly vague question but it's puzzling me

Why can I only care about/ focus on one thing in my life at a time

For example, if I'm doing theatre, all of my focus and rive and everything in me is focused on that and nothing else matters. If I'm doing music everything goes only into that. If I'm even gong into my crappy job on campus and have a reason to like being there, all of my attention and everything in me is focused on that. But never have I cared about all 3 as passionately at the same time..if that makes sense.