When you come to the brink of something and you just almost fully reach it, but don't. Also the fact that every single individual person will never learn everything they want to learn or see everything they want to see in life(most likely, depending on what they want to learn or see). For instance, no one can read every single book in existence and accumulate all that knowledge. No one can visit every city in existence in one life time and gain experience from every individual city in every country/state/province/prefecture/etc. There is a limit to what one person can do, see, and experience in their lifetime. You have to take what you can from your own life and cherish what you can; whether that be memories, experiences, what knowledge you accumulate, what relationships you have, what you see, what you do, and what you become.