hi jiwoo...
don't be sad, i'll answer for you
i really love the Beautiful Day's movie i watched it 6 times heheheheh...;>
the reason is caused u never fall in love and at the end of the scene Zero also says thay he couldn't make YuhnSu(JiWoo) laugh or smile.
and the reason why YuhnSu(JiWoo) and MinChul together forever is
1. They need each other and the depen on each other
2. They have a lot's of unforgetable memories than Zero
3. At the end YuhnSu(JiWoo) seen to be happy than before, cause she found
   who can give her happiness ( see the part that see said " I want to tell people in the world  
    Lee min chul Is my Boyfriend)
anyway the other reason is : it's only a movies so don't blame her, she's only doing her job and she has to followed what the producer want and write on the white paper (script/scenario)
ok, i think the reason is enough, there is to many reason to tell ehehehhe;>(Peace!!)

;>from the person admire u (jiwoo) (bunny)
take care