Competition is key - when it becomes competitive, then it becomes a 'sport', and bodybuilding can be competitive
otherwise, you're just reshaping your body for aesthetic purposes or leading a healthier lifestyle, in this case it would fall under 'training' and not 'sport'
someone here mentioned that bodybuilding is more of sport than any other.. you can't really make that type of argument
bodybuilding requires time, dedication and planning
something like soccer or tennis needs pure on the spot technique and skills, bodybuilding is a complex of simple tasks, whereas something like tennis or soccer is the mastery of very few subtle tasks, two different but equally challenging things
anyway, point is, bodybuilding is an athletic activity and it can be competitive and does require considerable effort and knowledge, so it is a sport | 
