I started following the Red Sox around 1971. Didn't real know or care much about the Yankees at the time, as the Orioles and Tigers were the AL teams to beat at that time. The 1975 team secured my lifelong fandom, but the third place Yankees were not really on the radar that season (the Orioles were the team that were the biggest threat in the division that season). I neither liked or hated the Yankees; I was just ambivalent.
The only resentment I had for the Yankees at the time was the fact that they acquired Sparky Lyle from the Red Sox for one Danny Cater. I never understood that trade (still don't, as I never heard a rational explanation for it), but blamed the Sox front office instead. I remember the Yankees signing Catfish Hunter at the end of 1974, and I didn't quite understand why the Yankees were able to get such a good pitcher and the Red Sox did not. Of course, Steinbrenner was quietly building a solid roster via some trades and drafting, but I was too young to pay attention to that.
It all changed on May 20, 1976. There was a ton of hope for the 1976 Red Sox after the team had come so close to beating the Big Red Machine the prior October. For the first time in like forever, the Red Sox acted as a big market team and traded for Fergie Jenkins just a couple of short weeks after the World Series, who would join a rotation of Luis Tiant, Bill Lee, and Rick Wise to anchor what was hoped to be one of the stronger pitching staffs in the AL. The same lineup that led the AL in runs scored in 1975 was returning intact, led of course by the gold dust twins of Jim Rice and RoY/MVP Fred Lynn.
Instead, the Sox stumbled badly to a 6-15 start, which included a horrific 10 game losing streak that began late April. Jenkins started at 1-5, seriously victimized by a total lack of run support. Rick Wise had an unsightly 6.62 ERA. Bill Lee was even worse, going 0-3 with a 9.27 ERA. Tiant was his usual excellent self, going 5-2 with a 2.57 ERA, and almost single handedly kept the Sox from falling deep into the abyss. And the lineup unexpectedly regressed at the plate.
However, the Sox started to appear they could turn things around. Going into the series against NY, they had won 7 of their previous 8 games to pull within 6 games of the AL East leading Yankees (who were off to a scorchingly hot start). Bill Lee was cruising, scattering 7 mostly harmless hits over the first 6 innings as the Sox had a 2-1 lead. A collision at the plate between Carlton Fisk and Lou Piniella was followed by a bench clearing brawl, during which Graig Nettles and Mickey Rivers teamed up against Bill Lee and threw him violently to the ground, dislocating his left (pitching) shoulder in the process. Lee was expected to miss the rest of the season as a result, and the league did nothing. And while Lee did return that season, he never was able to regain the form that won him 17 games each in 3 consecutive seasons. From that point on, the Yankees were public enemy #1 in my eyes.
The Yankees signed Reggie Jackson in the subsequent offseason, and then 1978 happened. I don't hate them as much as I used to (2004 cleansed a lot of sins), but I'll always smile when the Yankees lose.