On the one hand, Marco Reus was one of the players who was quite disappointed at the end of the Dortmund vs Mainz match because he really dreamed of a Bundesliga trophy and he failed to get it.
He was came to Dortmund on 2013 which mean one year after Dortmund became an Bundesliga champions and when he comes to this team Reus was hope he can gets the trophy with Dortmund because at that time Dortmund were very solid under Jurgen Klopp but in fact Dortmund indeed has been several times to compete for race title with Bayern but they were never can make it again
And this season when Dortmund were very close to the title but in the last match Dortmund were failed to beat Mainz which mean Bayern automatically is the champion of this season so it's normal if he feeling dissapointed because during his career with Dortmund all he want is to get this title and for next season it wouldn't be easy because i am sure Tuchel will be evaluate Bayern performance and they will be more solid than now