But there are rumors, that he already had an offer from Barcelona last year. When it is your dream, why go to another club?
And it is very naive to think that his prime or even sole motivation is his love of Barcelona. That is something, that only fans of tier 1 clubs can believe. Because they pay their players the highest wages in the world, so that never becomes an issue. Dembele will at least double his wages with his move.
By that logic Reus just plays for Dortmund because he loves the club. Has nothing to do with the being the top earner at the club and the wages (8.5m plus bonus)...
The decisions of a player are not simple as that and have usually nothing to do with any love for the club. It has everything to do with the money he earns, the status of the club (title chances) and the role of himself in the team.
And for Dembele it is more about the status Barcelona has in the world when he grew up. How could you have a connection with a club of a city, where you and nobody in your environment (friends and family) has ever lived and you never watched in person at a football match?