All of us want to be eternal. Some may not wish to actually live forever (often this thinking is associated with the body and relationships we currently have) but all of us crave it in some fashion or another. The concept of legacy is very much related to eternality. Even if this material body ceases to exist, we all want to leave something behind and be remembered. Whether it be children, accomplishing one's goals or somehow affecting other's lives in a meaningful way, this concept of eternality is within us because it is who we are.
The soul is eternal and we are the soul. Due to the fact that the soul is wearing a material body, we easily forget that we are actually eternal and identify ourselves with the body which undergoes birth, disease, old age and death and which grows from boyhood to youth to "distinguished (aka: old)." :D
In this verse, Krsna gives us supreme comfort. He comforts all of us by saying that we have always existed and need not fear death. This is really powerful. Modern society tends to ignore death and hides it behind lots of other things. It is interesting to note though that many religions and spiritual traditions, including that of bhakti, speak of death in a matter of fact way and address it head on. They do not bring in the negative aspects, but instead focus on the positive. Some may question, "Is there anything positive about death?" The bhakti texts loudly answer by saying "YES!"
By being aware of death we can reflect on how we utilize our time. Our time is spent in so many ways whether it be in performing our duty, honouring commitments, having fun or just plain wasting it. For how many is spiritual life an actual priority? Sadly to say, not that many. But this awareness can help shift one's priorities. This doesn't mean one should neglect one's duty, family, job etc.., it just means that spiritual life has it's rightful place in one's schedule.
As well, this verse speaks of how loving and compassionate God actually is. Here Krsna reassures Arjuna by saying there was never a time when both Krsna, Arjuna and everyone else didn't exist. Krsna is saying here that, "My relationship with you is also eternal." It's not that we are eternal and don't have a relationship with God. No! Our relationship is eternal too. This goes to show how much God loves us. He will never give up on us and our relationship with him (whether we acknowledge it's importance or not). How lucky are we to have such a Lord that loves us unconditionally?
Remember that next time you feel unloved and down in the dumps. The Lord loves you more than you can imagine and is just waiting for you to remember him.
