Help MyTouch 4G hotspot turns off, why?


I have a T-Mobile MyTouch 4G and am quite happy with it so far. I enabled the hotspot and just let it run to see if it affected any other activity. When I checked back, it had turned itself off. Maybe 30 minutes between checks. I cannot find anything yet on the internet or here which sheds light on the question. It may be it turns off if not used for a period of time, just a guess. Does anyone know the answer?
Question is why would you keep it connected if you don't use it? just turn it off. I turn WiFi on/off depending on when I use it and when I don't use it even if its idle then it still use alot of battery.