Board Thread:Naruto: The Last Discussion/@comment-24098614-20150126075513/@comment-22634052-20150126093610

*Spoilers* Before Hinata agrees to marry Toneri, he had captured Hanabi and later takes Hanabi's eyes so that he can awaken the Tenseigan and destroy the Earth. He takes Hinata to his fortress and she explores the place and tries to find a way to stop him from destroying the planet, which was probably her main reason for agreeing.

As for why Naruto is so depressed after Hinata agrees to marry Toneri, it's probably both - Hinata and Naruto are on this mission to rescue the kidnapped Hanabi, during which Naruto realises that he's in love with Hinata. Naruto confesses, but immediately after he does so, Toneri appears and Hinata agrees to marry him without saying a word or responding in any way to Naruto's confession. Ouch.

... Yeah, he's pretty clueless about love and it took him that long to figure it out.