Don't we value life precisely because it is fleeting?
Advocates of aging say that a contrast such as sadness and joy, failure and success, rain and sunshine or night and day is always needed to make a particular thing valuable. That is why, they say, the value of life lies in its transience. However, it is life that gives us the opportunity to perceive these contrasts in the first place. Without life, these contrasts - like everything else - are meaningless to us. Since we do not notice our state once we are no longer alive (and, at least so far, we do not come back to life afterwards either), there is no such contrast here. In order to be able to appreciate life because of the contrast of death, we would have to experience death and life alternately (as with bad weather and sunshine, for example), which, however, is not possible for the reasons mentioned.
And this even accepts the assumption that it really is the contrast that makes something valuable, and we would be very careful with this assumption. In our view, for example, joy does not become valuable because sadness exists; it is already valuable in itself.
To answer the question, we should ask ourselves why we value our lives today. Why do you value your favorite food, a particular book, sunshine, friendships, romantic relationships - or whatever it is in your case? Our strong suspicion is: because you like these things.
We do not value life and its joys because we know that, as things currently stand, it will be over in a few decades at the latest and that we will therefore at some point no longer be able to eat ice cream, go swimming or be with the people we love, but because we enjoy living and like certain things or activities.