Use 'preoccupation' when referring to someone's hobbies or interests that take up a lot of their attention. His constant preoccupation with model trains left little time for other activities.
Pro Tip 2/3
Not Always Negative
'Preoccupation' doesn't always mean worry; it can be a neutral term for what occupies the mind. Music was her main preoccupation, filling her days with joy.
Pro Tip 3/3
Professionally Used
It's appropriate to use 'preoccupation' in professional contexts to describe focus on work or projects. The team's preoccupation with the deadline ensured they finished on time.