why am i obsessed with calories counting ?

I'm 15 years old and 5 foot 3inchs at the begging of the year i was nearly 8stone and im 7stone now i have this app on my iPhone called my fitnesspal and i use it to count my calories and how much exercise i do everyday and using this i have lose weight, i always make sure i have burned twice the amount of that i eat or more so it's like im eating nothing because i burned it all off and i make sure i eat under 900 calories everyday.