It's the fanbase - people that call themselves Cubs fans because it's the "cool" thing to do.
I do have a natural reason to hate them - but I hate their fans much more than I hate members of their team. I like Lou Pinella. Dempster is one of my favorite non Sox. It's the fans...
I must spread some reputation around before I can give it to you again. However, you nailed it.
I was at a Cards/Brewers game in StL earlier this season, and there was some d-bag in front of me who had on his Soriano jersey over a wifebeater, a cubs hat turned sideways, a braided belt (not kidding), a giant gold chain, and pants sagging at least halfway down his thighs. He was one of the most obnoxious fans I've ever encountered, and here he was at Busch stadium decked out in Cubs gear when they weren't even ******* playing! In fact, it was two division rivals, and here he was in Cubs gear, going on and on about how this is the year, etc. I wanted to slap him and inform him that they can't both lose, so what the hell is he hoping for??? I've been to Royals games, Astros games, etc., but I never wear my cards gear to their stadiums. The only time it's appropriate is when your team is playing. That's it. Otherwise, you're just a d-bag.
I will say this though, I do love a good Cubs/Cards game in St. Louis. I've probably been to 5 of them, and the majority of the cub fans really aren't that bad. But, that said, there sure are some bad apples that ruin it for the rest of you guys.