The opening scene alone is enough to make you weep. The song in the beginning is so melancholy...*sniff*
When Arthur and Merlin turn back into people after being squirrels and you hear the female squirrel who fell in love with Arthur sobbing hysterically. Man, it's hard to even think about without tearing up.
Merlin: Ah, you see, lad... that love business is a powerful thing.