Dortmund achieved nothing and there is no reason for them to be happy after this year.
In fact, they lose what they could get easily and maybe even years after that they won't have this chance to have a close race with Bayern Munich and have Bayern Munich in this situation.
I also think that the outcome of this season was very tragic for Borussia Dortmund. The sad reality in the Bundesliga is, that other teams only have a chance to become Bundesliga champion if Bayern Munich is playing a very very bad season for their standards because otherwise they are just to strong. This season that was the case and still Borussia Dortmund somehow managed to screw it all up in the very last match. I think they won't get another such big chance again so quickly.
It's just that Dortmund wasn't ready to become a champion, so many supporters in the last game to celebrate Dortmund as a champion that the stadium was so full as to make Dortmund players awkward and don't have a winning mentality, they should have won their last match and sadly were held to a draw, the champion was in front of their eyes but Dortmund did not take advantage of that opportunity which may not necessarily get the same opportunity next season.
That is why a winning mentality is needed, not only having good players in the squad, the most important thing is mentality, sometimes mentality always goes up and down in a condition, this will be a valuable experience for Dortmund and maybe it should be better next season to won the league. Dortmund only lost on goal difference and it's not really that embarrassing, it's just that it might disappoint all the supporters.