Because Earl the Cat made Haru feel good: Haru likes Earl the Cat. When the person she likes breaks up with her previous girlfriend, of course Haru isn't going to be happy about it. Haru became a cat squint because she lost herself and wanted to be a cat squint which is not bad. But in order to turn back into humans, they (Haru, Earl, and Chubby) must climb to the top of the highest tower in the cat kingdom before sunrise. After a battle, they are about to reach the top of the tower when it is blown up from the bottom by the king of the cat kingdom. At this point the prince returns. He says he wants to marry Yuki, not Haru. The King of the Cats again wished for Haru to marry him, and it was the Count of the Cats who made Haru come back to herself. As for Haru's smile at the end, lol, I didn't pay attention.