Examples of why

We are on a journey that can be described in various ways, and our belief system makes a profound impact on how we are going to approach it.  If we believe that this life is all there is, then we are on a journey from birth to death, and we need to maximize our experiences of life and love along the way so as to miss out on as little as possible and make the biggest difference that we can.

Christian faith brings some similarities, but profound differences. One similarity can be found in the parable of the talents: we are clearly to make the most of what is entrusted to our care. However, even the "maximizing" that's implied there is very different from that of the previous approach, which tends to be for the benefit of our own experience, glory and posterity.

Considering the differences in believers' vs. unbelievers' understanding of the nature of our journey illustrates even greater differences. Rather than traveling from birth to death, Christians believe the opposite: we're traveling from death to life. This world offers only the former, and we believe that Jesus Christ alone delivers the latter. This provides a completely different scale by which to measure every experience opportunity along our way: will this thing bring me and others closer to eternal life in Christ, or move us further away? We also express the transformation as being from darkness to light; from egocentrism to real, self-sacrificing love; from sinfulness to holiness, from slavery to freedom; from self to the very image of Christ. But however we view the journey, we are to have a clear vision that where we are headed is incomparably better than what we have left behind. This becomes a matter of trusting in God, and the scriptures give us several illustrations of what happens when we long for what is behind us:

When God spared Lot's family from the destruction of Sodom, his wife could not resist the temptation to look back on the home they had left. BTW: now there's a scripture passage that I have a hard time interpreting literally. Still, the image is clear: we must trust in God that the way he sets before us is better than the way we have left.

When the children of Israel grumbled against the Lord and Moses for bringing them out of Egypt because of their lack of food and drink, they were set upon by seraph serpents, from which the Lord delivered them by means of a bronze serpent lifted up on a pole. Jesus himself referred to this as a presaging of his crucifixion. Likewise, the people were chided at Massah and Meribah even as the Lord provided the water they truly needed. (etc.)

Jesus said that whoever sets their hand to the plow and looks back is unfit for the kingdom of heaven.  As a practical guideline: I suppose it's nigh impossible to plow a straight row while looking backwards.

All of these examples illustrate why it is counter-productive -- even useless -- to dwell on the things we've been called to lay down. If we keep score of such things in our relationships, we will impede our loving and growing together. If we cling to parts of ourselves that we are to shed, we will never be transformed beyond ourselves.