I don't believe you're being selfish at all. Your dog loves you and as long as he receives food, water, companionship and love, and gets outdoors he's getting from you what he needs. Perhaps your friend would be willing to come walk him or take him for a run every couple of days just for the near future when she takes her dog out (but that might be too much to ask). I think you give your dog more than you're giving yourself credit for.