To a Christian, the opening quote seems perfectly satisfactory. The caveat given Barnes colors the quote in a totally new manner. Why is there a difference? For Fletcher, love is arbitrary, relative to the arbitrary arbiter. It has a foundation in everything and therefore a foundation in nothing. It has no stability. It becomes pure multiplicity. It ultimately will fall apart like a poorly made deck of cards. For the Christian, love is based on a person, on three persons, in fact, Father, Son, and Holy Spirit. Deus caritas est. God is love. Love, for the Christian, has foundation in Truth, in Goodness, in Oneness. The foundation is rock solid.
These two concepts butt heads often within out lives in the world but not of the world. We might not be aware of the harmful consequences of the former understanding of love because it seems so pleasing and enjoyable. It lacks responsibility, which due to our first parents, we never really want to take (just think, "It was the woman" & "The snake told me to"). Furthermore, it exalts our weak egos lifting them to arbiter of our morality. We can make ourselves. These are what make the former understanding of love so popular.
The Christian understanding demands sacrifice. It calls someone to go out of themselves and be for someone else. It challenges to lay down ourselves in the law given to us by the creator. It moves us to responsibility for our actions, those times when we rejected love or failed to act in love toward someone.
* Hazel Barnes, The University as The New Church, C.A. Watts & Co., 1970.