Luke 9.62: "No one who sets a hand to the plow and looks to what was left behind is fit for the kingdom of God." Jesus was referring to how we should follow him. Looking back upon our past, no matter how pleasant our memories, stops us right in our track from following him. For us, the best is yet to come, so there's no point in going back into the past. The good disciple doesn't wonder about the future either because he is already with Jesus, his future. Staying with him every step of the way is being secured for all eternity.