They played eachother in the playoffs a bunch of times in recent memory. Playoffs are where rivalries come from.
2002
2004
2008
2009
2011
2014
+the Chara incident
Also Wings/Leafs sure, original 6, but they only played against each other 2 times every year for the past 20 years or so.. there is no on ice rivalry right now. Caps/Pens I still don't think was much of a rivalry outside of Ovie vs Sid media hype. Therre was that one playoff series that was insane but thats it. Pens/Flyers is an actual rivalry based on what happens on the ice.
To add to the Habs-Bruins rivalry...
In the 50's, when Richard was hit and suffered a massive cut, it was against the Bruins, by a player called Leo Labine.
When Richard was punched while a referee was holding him, it was by Hal Laycoe who was playing for the Bruins.
When Zednik (almost forgot he was playing for the Habs) got an elbow in the face in 2002 and miss the remainder of the playoffs, it was by Kyle Mclaren of the Bruins.
In 2004, the Habs beated the Bruins in the playoffs 4-3, after trailing 1-3.
The rivalry always existed, but it got worse with the Patches-Chara incident.