Why Everything ends Sad :(

I really wonder, why things are tough in the beginning, happy.. really happy in middle and turns out to be so sad at last sometime, as far as i know many times....

The above are the Movies which i felt really bad after watching. The first movie

Simon Birch :
I would call this movie as a master piece, I wont tell you the story but, how i came to know about this movie is really a funny story. I am a Jim Carrey fan, and it was my semester holiday. I planed to watch all Jim Carrey movies. I went to IMDB and searched for his works. Ive watched all his movies including "Simon Birch". And while watching this movie, i came to know that Jim is just a narrator and even though he has a character in this movie, he come only in starting scenes and at end only. I felt , ok , why should i watch it. But when the movie went on it was fun, the small boy named Simon Birch, and his friend Joe Wentworth,. It was full of comedy and little tragedy in the starting but and end,. oh no ,. oh no.., i was watching this movie alone in my room that night,. I hate to say this ,...... i cried and the water came from my eyes couldnt be stopped for atleast for 10 mins,.. then i felt really bad and went to sleep.

Bridge to Terabithia
What can i tell you,. an other tragedy .., real serious movie.. well i can speak abt this movie., I felt more myself in the boy who is the hero of this movie. Damn, i use to be alone like that., and suddenly a friend, a girl friend comes into his life everything changes ,.. and she leaves.., how could it be,. my words couldnt bring the real picture but i felt like my own girl friend died and i was crying ..,

A Walk to Remember
Well another movie,. attracted by its poster actually i watched this movie,. a death in the beginning,. oh no is it like 'Saw' or 'Final destination' , i dont like those movies, should i watch this..., this was the Question on my mind,.... movie went fine,.. fine,.. happy.., happy..., excited.., sorrow..., terrible sorrow,... death..., i should have watched 'Saw' or 'FD' .., And its a love story.., sad love story.

My point is why even imaginary stuffs too aren't good ?.., First things are new, and then it becomes familiar and finally it lasts no longer .. why why why.., Why shouldn't Simon be alive, why not Lesli , and why not Jimmy.., ... tell me a answer if u find.