What does it mean to be 'in love'?

To be in love is not necessarily to love. To be in love is a state; to love, an act. A state is suffered or undergone; but an act has to be decided upon. Now, the promise which marriage means cannot fairly be made to apply to the future of a state in which I am at the moment, but it can and should mortgage, the future of conscience acts which I take on – to love, to remain faithful, to bring up my children. That shows how different are the meanings of the word ‘to love’ in the world of Eros and the world of Agape.

Denis de Rougemont, Love in the Western World