On top of the different epilogue messages depending on which levels you finished, there are also 4 different Happy Tree scene endings depending on your mood (the score).

Ending	How to unlock
Ending 1	Get over 30,000 points.
Ending 2	Get between 20,000 - 30,000 points.
Ending 3	Get between 10,000 - 20,000 points.
Ending 4	Get less than 10,000 points.