>>48911662An additional, pseudo-Buddhist extrapolation I tacked on to the above was that Homura's experience with magic has made her effectively immortal baring damage to her soul gem. However, she is bound to her role as a magical girl either out of a desire to protect Madoka's desired world, or to work off the Karma she accumulated during the time loops and dumped on Madoka. From the little I know of Buddhism, these last 2 points may not have to be separate.
Granted, this line of reasoning has nothing to back it up, since it's mostly born out of my personal desire that their eventual reunion be more than just Madoka coming to punch Homura's ticket and the end of it all.