I don't think it's true that we leave nothing behind. For a start, there are memories. Then there are the written words that have survived milleniums; think the Bible and other religious scripts. Before the written word were cave paintings and carvings, Then there are scientific discoveries. The person who discovered the function of the wheel may be unknown now, but what s/he left behind has been vital to humanity as we know it.

Yes, we have a lot to regret, too. Problems that later generations may have to solve, or at least adapt to.

As to the rest, though, I think you are correct about life being a gift to be appreciated. It's a shame most people don't realise this when they are young.