Quote an interview by Excite:

Excite: So as seen in your lyrics, "you look up to the lights in the house; you look at the sad faces reflected in the puddle of water". These little actions by the character (in the song) actually gives the idea of the whole story (to the listener), that is difficult to express in words.
YUI: Also in the lyrics, "I tried saying I'll be fine. But that's not possible." The word "fine" (daijyoubu) is the key word. When someone asks you whether you're fine, the natural reply will be "Yes I'm fine", but when you cool down and think about it, you'll realize that you are actually not. (How can it be possible?) This is what the main character in the story feels. As a detective (the main character in the drama), it's inevitable to feel afraid or uneasy at times, but one always tries to deceive himself by saying he's fine. But the truth is otherwise, isn't it?

Excite: How about you yourself?
YUI: Of course I feel the same way, like everyone else. Taking a couple/ lover as an example, when both of them are parting for one month (say, for work), when the guy asks the girl whether she'll be fine. Her natural reply would be "Yes I'll be fine. Go for your work." Isn't it so?

Excite: Yes, indeed. She'll try to act tough. (to assure her husband/ boyfriend)
YUI: Yes, a phrase that we always use "I'll be fine", "Don't worry about me". It's something that's said too easily.

-Credits to depyon

That's so true, ain't it?