^My guess, after google-translating the lyrics.

The tree probably represents her determination in keeping the relationship going, while the sorrow and tears make this determination grow stronger.
Course, they might've just thought it was cool for a tree to be in a room.

I like Cry much more than Tsuki though. Tsuki was too cool for my taste.
And maybe it's because Cry sounds a little anime. It would totally fit as a "Tales of" RPG opening song.
