In my sophomore year of high school I remember having to read The Epic of Gilgamesh as part of our required reading assignments. This Sumerian epic poem dates back to 2100 BC, is widely considered the world's first great work of literature, and has an abundance of important spiritual lessons "encoded" within it's words. In particular, the quote: "there is no permanence". A simple, yet complex statement, that applies to life on many different levels.
A vast majority of us go through our lives holding on to situations, emotions, memories, and people that no longer serve our highest purpose. We build up ideas, and cling to these ideas with all of our might because the very notion of having our illusions shattered is devastating. We operate on an "end-all-be-all" frequency, and the primary goal of "this is it" is actually a very toxic way of thinking. It's the same Santa Claus principle of childhood applied to adulthood...You spend so many years believing in something, only to find out that what you believed was only an illusion. First it's sad, but then you eventually learn to let go.
Letting go is one of the most important tools we can keep in our spiritual "tool box" of life. When you fully accept that "there is no permanence" and every situation, emotion, memory, and person that comes into your life is there for only an allotted amount of time, it allows us to remember to stay present, and focus on the here and now. By clinging to the past you aren't allowing yourself to be present, and by worrying about the future, again...you aren't allowing yourself to be in your full present presence. Unfortunately in life, no matter how much we wish for forever, forever does not exist. All that exists is now.
We will continue to experience lessons throughout our lives that force us to evolve, ascend, and discover our ultimate truth. When we can let go, and appreciate all experiences, good, bad, or indifferent, we essentially become spiritually invincible.
Letting go allows better experiences to flow, and finding appreciation of the fact that "there is no permanence" will allow you to remain in your present presence at all times...and that truly is a gift.