Unguided logic

The promise of love is what keeps man moving forward.  Finding that love is what helps them cherish the present. Losing  love is what keeps them looking back and yearning for what they once had. 

What is the fundamental fallacy in those actions?

The promise of love should keep us looking back only to understand how far we've come. That losing it should make us move forward to find it once again; and finding it isn't about cherishing it but rather nourishing it  so it grows beyond our dreams.