Why would a mother not tell her daughter she loves her?

my mom and i have had a very difficult relationship. I am not a raging drug addict even though she thinks i am. I promise you I am not. Yet whenever I do visit my mother I always tell her I love her, every single ime, yet she never responds. She no longer tells me she loves me when we part. Why does she not say it back to me? Doesnt she realize how bad that hurts me?