This is a quote from the movie 'Lucky Number Slevin'. I believe the idea is the same to answer your question :) Not all of it is relevant though :L 'The unlucky are nothing more than a frame of reference for the lucky, Mister Fisher. You are unlucky, so I may know that I am not. Unfortunately the lucky never realize they are lucky until it's too late. Take yourself for instance; yesterday you were better off than you are today but it took today for you to realize it. But today has arrived, and it's too late... You see? People are never happy with what they have. They want what they had. Or what others have...I'm a bad man who doesn't waste time wondering what could've been when I am what could've been and what could not have been. I live on both sides of the fence, and the grass is always green.'

Because the human being is stupid and egoist. We don't know to appreciate something until losing it

cuz it has contrast between yesterday and today. yesterday we had something while today we lose it. so quickly that we haven't prepared for it. and then,we start regret. we always think about the ifs. if we have,if we can.... so,seize the day and don't regret for it.

coz when we were having it we didn't enjoy enough of the good things coz we know we could have that happiness but we didn't do and when lost it just feel regret. its coz its a thing we r sure we could do but we didn't then realize the value