Why did it take Hortons injury for the Bruins to play this great?

I know the Bruins aren't the better team overall, than Vancour or even Tampa Bay for that matter.....but when they play BRUINS hockey and get their emotions going in the game, their isn't a team that can contain them, or come out on top against them when they do, idk why they don't always play every game like this....it seems only when they are with their backs agains the walls they do