"Thanks to you, I can laugh again."
Translation:Dankzij jou kan ik weer lachen.
That's true, normally you can use both "je" and "jou" as an object (or after a preposition like "dankzij"). But because of the meaning of this sentence, it is wrong to use the unstressed form "je". The use of "dankzij" automatically puts emphasis on the pronoun.
This is quite tricky and an understandable mistake for language learners.