We are not commanded (or forbidden) to love our mates, our children, our friends, our country because such affections come naturally to us and are good in themselves, although we may corrupt them. We are commanded to love our neighbor because our natural attitude toward the other is one of either indifference or hostility.
Love never dies a natural death. It dies because we don't know how to replenish it's source. It dies of blindness and errors and betrayals. It dies of illness and wounds; it dies of weariness, of withering, of tarnishing.
We really have to understand the person we want to love. If our love is only a will to possess, it is not love. If we only think of ourselves, if we know only our own needs and ignore the needs of the other person, we cannot love.