Why do we group love into some binary function — on or off?
We do this because one or both lovers desire exclusivity, the kind that requires commitment, and commitment requires sacrifice. How that exclusivity is understood is influenced by social conventions as well as the personal trust and security of the partners.
The enterprise is made complicated by a lack of self-knowledge and by dishonesty. The young man who feels the onrush of sexual desire may believe in the moment that he is in love, or that he “loves,” (in the same sense a woman may love chocolate) but once the desire is satisfied, other feelings arise, which may include fear of what just overcame him, regret of saying things he no longer sees as true, as well as extreme embarrassment over revealing this tyrannical and demanding part of himself. Adolescent males tend to either identify with that drive, or to attempt to disown it: the former are unashamed of shameful behavior, the latter are inhibited from socializing. The former too often regard lying as a justification for satisfying the sexual urge (“All’s fair in love and war” shows the confusion of “love” as desire with love as sacrifice), the latter have not assimilated their drives into an integrated personality. In fact, in the days of arranged marriages, the exclusive partner was thought to be necessary for that achievement.
You are quite right to point out that love develops by degrees. What you are willing to give up for the beloved shows your degree of love. I was willing to give up all potential lovers for for the woman who is now my wife of 40 years. She was willing to share an uncertain future with me under the same conditions. We are in love.