One of my friends recently had this question on her blog. This is my answer to her post.
Love = Happiness is a differential equation because like you mentioned true love is expressed in many forms. Be it holding hands, looking into your partners eyes, remembering some trivial thing that might mean a lot to the other person, writing something inspired by them, giving them little surprises, writing a few love notes etc. Each in turn goes a long way in making the other person happy which makes them fall in love with you even more. Hence proved.
Now coming to the part about why Love is not a constant. If love were a constant, it would mean doing a certain set of things something in some pre-conceived manner to make someone fall in love with you. Which unfortunately is what seems to happen nowadays. Love is not some ‘constant’ formula that needs to be applied whenever you want someone to fall in love with you.
Hence love is an integration of the little differential equations of joy that go towards making someone happy which ultimately results in true love being attained.