What we do, what we achieve, what we feel, what we experiences remain relevant till our death. Once we die, all become worthless, useless. But our good deeds helpful to the society, human kind or to all living beings pass on to the next generation.
After listening the words, Arjuna changed his mind set for leaving the battle for the sake of lives of his relatives, Lord Krishna told Arjuna that you are talking like a wise man but you should know the qualities of a wise man and recited as follows:-

श्री भगवानुवाच

अशोच्यानन्वशोचस्त्वं प्रज्ञावादांश्च भाषसे।

गतासूनगतासूंश्च नानुशोचन्ति पण्डिताः

So the literal meaning is that Arjuna, you are grieving for those who should not be grieved for and yet you are speaking like a wise man. The wise men do not sorrow over the dead or the livings who would die one day. The intrinsic or inherent meaning of the above couplet is that the wise person never repents the past nor worries for the future; because, the wise man knows the importance of the present. Most of the people spend their precious time by thinking about their past failures and
experiences. More amazingly these people who reside in their past have two options i.e. either they boast their past achievements or cry over their failures in the past. Similarly there is another group of people who always stay in future and remains worried for this. Either they spend their time in day dreaming of future events or worrying about a beautiful and prosperous future. They visit to oracles, fortune tellers, palmists, to know their future. Both the groups of people have one thing common and i.e. they forget to work in present. They are worried for the things or situations which are not in their hand. Past happenings are like a book which cannot be altered and future happenings are like imaginations which need not to be true. The only thing which can be changed or used properly is the present. So here the problem is that when we think about our past, we invest our brain and its resources there. So if we were traumatized in the past then our brain will try to create the same situation and same feeling of that instance. So, our body will start to secrete the hormones like stress hormones and others. So, due to this, the energy, focus of a person are lost and he/ she remains trapped inside his/her illusion of past. Similarly, if one is always concerned about his future without working for the same, his brain tries to bring experience for him through imagination and collaboration of the past happy experiences to it. Here also the brain and the resources get engaged in creation of a false happiness. And the hope and probability turn into just a dust of day dreaming.

As per Lord Krishna wise people neither believe in past nor in future. They act in present. Those who are dead they are already gone and we cannot do anything for them. Similarly, those who are alive will die one day, then also we cannot do anything to stop their death. So it is natural truth that who had taken birth would die one day. If we keep our mind busy in mourning f those who had dies or by thinking for those who would die in future, we would not be able to do our duty, our Karma. It reminds me a famous proverb: – Yesterday is a history, tomorrow is a mystery,
today is a gift of God, which is why we call it, the present. There is a very beautiful story in this regard. In a kingdom, there was a poor man living in a small village. He used to beg to meet the need of his life. One day he went for
begging. A noble and rich man donated a pot of milk to the poor man. After receiving such unexpected gift, he was very happy. He carried the pot of milk over his head and marched back to his home. On his way to home, he started to imagine how he would use the milk. Slowly his imagination turned into day dream. He dreamt that he would divide the milk in
two parts. One part he would use for curd and the other part for sweet. Then he would sell both curd and sweets. Whatever he would earn from the selling, he would purchase a cow. The cow would give birth to a calf and he would have an ample amount of milk to sell. One morning he would discover that the calf would leave untied. And it would run to drink all the
milk of cow. So the poor man ran to catch the calf. Suddenly he heard a sound of “Dham”. He discovered that the pot full of milk was broken by falling on the ground. When he left his hand from the pot to catch the calf of his dream, the pot fell on the ground. There are many things to learn from the story. Here the pot full of milk is our potential, our capabilities at present which is in our own hand. But we forget about that due to our day dreaming for the future. Same like the poor
fellow of the story we spend our present in day dreaming of future, when the time passes we find that we have ruined everything. So Lord Krishna advised Arjuna not to be emotional with the past or to bother with the result of the future. He stated that this is the real nature of wise people which help them to stay in present.