Simple, but good. It got even better once Tír na nÓg was released with that piano (Read the lyrics section.)

The song gets more instruments and more emotion as it goes on. It evolves perfectly.

Incredibly emotional. I don't even need to explain it. Go listen. Try to tell me it isn't emotional.

The early parts are kind of low on it, but it increases as time goes on.

It wasn't until Strive, __ years later, that these lyrics made sense. This is a song about Asuka and Sol (And maybe Aria). After all, he left for a space base at the end of Strive. And, of course, Tír na nÓg released, prominently featuring a piano on it (Specifically on its third area, which also happens to be the best area in the entire series). The picture of Asuka singing this, alone on the moon, to Sol, makes it even more meaningful and emotional than it already was.