Believe it or not, there was a time when I really didn't care either way about the Mets. In fact (gasp!), back in the mid-80s I even followed the Mets because they were such an intriguing team to watch. I never really "hated" the Mets back then and they were fun to watch. At the end of the day, my heart still belonged in the Bronx. I was just indifferent at the time.
Of course, it didn't take the Mets long to get back to doing what they do best - sucking. I could not believe that after winning the World Series in 1986, GM Frank Cashen began IMMEDIATELY dismantling what would have been a potential dynasty team. He couldn't leave well enough alone. One by one he picked them off - Ray Knight, Kevin Mitchell, Len Dykstra - while placing emphasis on spoiled crybaby types like Greg Jefferies. Watching all that go down reminded me why I was a Yankee fan, and after the '88 NLCS loss to the Dodgers enough was enough. I couldn't figure out why I gave two shits about the Mets anyway since I was a Yankee fan. The clandestine fling was officially over.
Still, I didn't "hate" the Mets. They were the NL team in town, good for them, they were bad, whatever. One day my buddy AJ said to me "there's no way you can be a Yankee fan and be ambivalent about the Mets". I argued this point but he said (in his prophetic way), "you'll see".
The answer to the question actually explains itself. What finally made me loathe the Metros was (drumroll)...Met fans. I worked with some pretty obnoxious ones at the time (a few were particularly obnoxious about it) and while I never disrespected or poked fun at them when the Mets invariably did what they do best (sucking), they never missed an opportunity to bust balls when the Yankees were going through lean times. It wasn't that they enjoyed watching the Yankees stink - they enjoyed busting balls of Yankee fans about it, which is pretty pathetic.
It's like Red Sox syndrome. When the Sox were finding new and exciting ways to lose every year, with their fans it was NEVER the Red Sox management's fault, never the players' fault. No, it somehow was always the Yankees' fault. The penis envy was always pathetic. Beer's too warm? The Yankees must have all the cold beer! Player made an error? The Yankees must have willed it to happen with voodoo! Turns out Met fans are just as bad, if not worse. No bad event is ever the Mets organization's fault, it's always those damn Yankees. Get a grip.
When the Yankees lose, Yankee fans don't walk around going "it's those damn Wilpons and the Mets - they get everything" (after all, the Mets have the highest payroll in the NL, so it's worth being envious of them, right...?). No, we know when they suck it's because they suck - not because some other team made it happen simply by existing (side note: somehow after finally winning a World Series, Red Sox fans have actually gotten worse about this...it's sad, actually).
The final straw was the day after Mickey Mantle died. It was a pretty somber day at my job, and the Yankee fans were having a group talk around my desk about The Mick and his unfortunate passing. On cue, the resident King Asshole Met fan came by and unleashed some of the most classless, scummy, childish insults about Mantle - and he did it just to dig at us personally so we'd feel even worse on a sad day. He almost got thrown out the 32nd story window, but cooler heads prevailed.
It was at that very moment that I awoke from my baseball amnesia. I realized that AJ was right - there was simply no way a Yankee fan could exhibit compassion to a Met fan because he would never get it back in kind. I had this proved in one trashy exchange that Monday in 1995 by a prototypical, "Letsgo Mets"-ing, Mr. Met bobblehead-ing Met fan in all his blue-and-orange glory. At a time when if the shoe had been on the other foot, I would have gone to this person and expressed condolences from one fan to another, I was not afforded the same luxury. At that moment I opened my eyes from the coma.
It was also the moment I went back to doing as I did in the 70s and wished for the Mets to somehow implode spectacularly. It took 12 years, but my prayers were finally answered, on the last day of the 2007 season. Even now, the thoughts of it take the sting off the way the Yankees' season ended. It was a fair trade.
So now you know. Never question the power of prayer.