Why do all good things come to an end??

Well I’m back again…a new day...a new question that I need to sort out...there’s a saying that goes—all good must come to an end...the question I’d like to ask is…WHY??...why do they need to come to an end??...why is it that if anyone has a good spell you find people telling them to “enjoy it while it lasts”…why is it that a good spell doesn’t last but if tough times come calling and decide to stay it is not that unusual…is it just because the world is so pessimistic that it thinks that good times don’t last??...or do we begrudge anyone with a better present than us that we become the sadists who ensure that the bubble is burst somehow, someway, eventually??...how can anyone be so sure??...i’m sure if we make the required effort they can and they will…or is it that we just don’t want to try?