The passuk hints to us an important lesson - the love that one can feel for physical pleasures - is always in the past tense - it's always fleeting. The feeling of joy that we may experience during a bite of delicious food fades even as we swallow it.
The love that Rivka Imeinu felt for her younger son had nothing to do with physical pleasure. When we experience emotions that are based on our intellectual capacity for recognizing and pursuing goodness - those are feelings that last. That's why they are forever in the present tense.