There are reasons why Real Madrid is the bookmakers favorite club to win the UEFA Champions League title this season and almost everyone who knows about the history of both clubs in the competition will agree to the fact that Real Madrid will always be the favorite team to win a final ahead of Borussia Dortmund. Like you said, Marco Reus who will be playing his last game for Borussia Dortmund in that game will do everything possible to try get his hands on club football's most prestigious trophy but I don't think his wishes will come to past in that game.
Real Madrid legend like Toni Kroos will also be playing his last club football game in that game and as a Real Madrid fan that I'm, I can authoritatively tell you that the Real Madrid players will do everything possible to help Kroos win that particular trophy in his last game at the club.
Real Madrid is undoubtedly a better team than Dortmund in every metric. They are also the most popular team in the world with the biggest fanbase worldwide. This fact is expected by bookmakers to attract many bettors to put their bets on Real Madrid. There is no reason for bookmakers not to put Real Madrid as favorite to win the Champions League trophy.
Dortmund may be underdog in this game, but their chance to win the final is still there. They have won one Champions league in 1997 by also being underdog to Juventus. The captain, Marco Reus, will put his best effort on his last game for Dortmund to win the most prestigious trophy for his boyhood club. The game will be a climax of this season football, it's expected to be an intense and attractive game that will lure many emotions.