Verse 4.5: The Personality of Godhead said: Many, many births both you and I have passed. I can remember all of them, but you cannot, O subduer of the enemy!
Many of us forget things from time to time. Whether it be where we left our keys, shut off the lights or checked the mail, there are plenty of things that fill up our head. It's no wonder that all kinds of apps and gadgets have been created to help us remember the long list of things that we hope to accomplish.
In the hustle and bustle of just trying to live our lives, it's easy to forget about the needs of the soul. After all, the majority of time tends to be spent on taking care of our body and things that are in relation to it. There's nothing wrong with taking care of the body, but just taking care of the body and totally neglecting the soul is counter-productive. That's because...
The soul is eternal whereas the body is temporary.
Most of us are well acquainted with the needs of the body, but what about the soul? In order to understand the soul's needs, one needs to learn about them from authorized sources. In this case, Arjuna has the perfect teacher, Krsna, to help him understand. To aid in that understanding, Krsna is explaining here that although the soul is eternal, it is limited.
Although eternal, the soul cannot remember the numerous bodies (i.e. births) it has taken.
Makes sense if you think about it. Many of us can't even remember what we wore yesterday, what to speak of trying to remember which body we wore in our last life or lifetimes prior! The question can be posed then - why is this important?
When we start to understand that we don't have the answers to everything because we are limited, it serves as a reminder that we are small. Sometimes the word small has a negative connotation, but in this case it just means we need some help. That help is available in the form of realized teachers who can help us understand who we really are.
When we try to sincerely learn and apply what the realized bhakti practitioners (and texts) teach us, we become empowered to understand their words. That knowledge becomes more than just theory; instead, we start to see their words manifest as reality in our lives.
So we don't remember our past lives or actions, big deal. If we stop and think about it - it might actually be a good thing! What's important is what we choose to do with our time now.