How did I not know that his song is going to be used as the theme song for JIN?????
As much as I like him and know it'd be an awesome ballad.... must say I'm kinda sad that they're not using MISIA's song again. It's the only song that could go with the drama for me.
I just thought how actually both songs reflect quite well the main character's emotions throughout the first season (well not sure about 2nd season, I have not watched it yet).
Hirai's song here is relatively calm, sorrowful and nostalgic, which reflects on the character's memories of his "past" (which is the present day him), whereas MISIA's song is much more theatric and dramatic and you can feel a sense of anguish in the song. This can be seen as the character's reluctance and how he is helpless in changing his fate in the whole time-warping situation...
This explanation is not very scripted so it's a bit messy lol