One whole nonstop week! Yay! So happy!
It was for the greater good of Thorin's soul. In DoS, Bilbo asked Balin if the Arkenstone found, would that help Thorin or hurt him? And Balin replied, it's best that it would stay lost. I'm not quoting it because I don't know the movies that well haha.
Bilbo probably had a hard time deciding because Thorin wanted it so badly. It must've caused him much grief to see his friend be unkind to the other dwarves because of the Arkenstone. So I think Bilbo did the right thing in keeping it away from Thorin. It would have made things worse.
What is your opinion on the matter?