#1. Both Bret Hart and Shawn Michaels refused to put each other over

Shawn Michaels and Bret Hart had one of the most iconic rivalries ever. Both were incredible talents and major stars in the WWF in the early 90's. The two feuded over the WWF Championship and had an incredible Iron Man match at Wrestlemania 12.
The feud became very personal on screen. Hart would call out Shawn Michaels' professionalism during promos and Shawn Michaels accused Bret Hart of having an affair with Sunny during the infamous "Sunny Days" promo. The tension got way out of hand and it all led up to the"Montreal Screwjob" It all started with Michaels refusing to put Hart over.
People argue that Bret jobbing at Wrestlemania 12 made Shawn's career. Michaels faked injury to avoid losing the title to Hart at Wrestlemania 13 and both men managed to avoid each other until Survivor Series.
Hart was the WWF Champion at the time and had just signed a contract with WCW the week prior. The WWE wanted Hart to drop the title to Michaels before leaving the company. Hart refused because he didn't want to lose the title in Canada and didn't want to put Shawn over.
Shawn, Bret and Vince agreed that the match would end in disqualification and thus Bret would retain the title. However, during the match Michaels put Hart in the Sharpshooter and Vince McMahon ran out to ringside and called for the bell. The bell rang, Shawn Michaels became the new WWF Champion and the "Attitude Era" was born.