shortest path algorithm – why backtrack from the end node instead of starting from the starting node?

I was watching a dynamic programming video by Erik Demaine . He says here , finding the shortest path by guessing the node after the starting node is not the right approach, and instead should guess the node before the last node. I didn’t understand his explanation. Can someone explain better why find the path backwards? It seems to me that you should get the same answer either way and both approaches are equally good.