The word 'torn' used here to indicate the difference between love and lust is not right IMO. If we are talking about relationships between matured men and women who are not related and they fancy each other..then these feelings are complementary to each other and work in the same direction with time. We know love or lust do change with time and depends on circumstances and bodily secretions of hormones. Usually, it starts with lust, goes on to a more stable form of attraction like attachment and finally strengthens to love. When lust hormones like testosteron or eostrogen decrease, secretions of other love related hormones also decrease affecting the feelings.