Well I'm not talking about obsession in the sense of a mental disorder -- but in the sense of that one thing X that a particular person happens to be in love with at the moment (could be piano, could be tennis, could be computer programming or could be something socially disapproved of like pokemon or skateboarding or whatever). The obsession creates a depth of knowledge. Even when you try to make such people do other stuff, they'll still be daydreaming about X. So better to work with it.