What type (breed or mix) of dog is he? (how much does he weigh - approximately)
How old is your dog?
Is he healthy? Is he on any medications?
What is he doing to request that his hips be rubbed? Does he want both hips rubbed?
Does he want to be rubbed and petted in other areas also?
Has he ever had any joint problems or lameness? Ever had any lameness in his rear?
Has he ever had any injuries in the hip area?
Is he itchy? How's his skin and coat - are they healthy looking?
What type (brand) of food do you feed him? Is he on any supplements or extras?
When you say "all the time", how often do you actually mean? i.e. how often does he request that you rub his hips?
Thanks for providing this information. It will help me to better answer your question.
I would say that she does this because it feels good. Basically, the same reason that people, including me, like their back rubbed. If her skin is a bit dry, it might feel all the better since rubbing her back would also make her feel less itchy there. Furthermore, that's a spot that a dog cannot reach so well themselves. Sort of like when I get an itch in the center of my back and I cannot reach it. She probably does it mostly at night because it's a time when you are both sitting and relaxing and she likes to become further relaxed by getting her to rub her back. If only our dogs could reciprocate and rub our backs - now that would be great!!! Then we could take turns!
Hope this answers your question. I don't think that there's anything complex in the reason why she does this. Enjoy your sheltie and your relaxing time together!