We know from experience that it is difficult to be sure. We do not always see clearly. It is not easy, in every case, to be sure of myself or my feelings and to have to depend on tangible proofs or signs.
Love is not like an idea with a definition or like a physical phenomenon that can be measured : Love is a matter of choice. And so, to use a quotation from Saint Bonaventure : Love is its own measure and standard.