I love the atmosphere in this. The warm lights that surround your characters work as really interest leading lines towards your characters. I wouldn't mind seeing the shadows be darker/more prominent. Also, the perspective on your characters vs the background is a little off. You should be seeing more of the tops of them rather than their sides. But other wise, great job on conveying the emotion of the piece.
Without knowing anything about it, I can tell that either they're close (romantically or otherwise) and spend a great deal of time together, perhaps anything is fine, just as long as its together. The warm lighting of the lights and brown tones of the trees gives that warm, loving feeling and helps set a mood. What I would have liked to see however, are more items in the house to make the place look more personal and lived in; perhaps a blanket to lay over, and other items that they might use in the treehouse.
Overall, however, its a good, warm, loving picture, and that's nice. Good job.