Why do people love crossovers so much?

This is an interesting discussion I had with a couple of buddies at the coffee shop tonight... Why do people love crossovers so much? I love them myself... it's fun to see Batman meet Superman, for example.

Not only would I like to investigate what the appeal behind crossovers is, but I'm also interested in what crossovers ATS/BTS members would like to see that hasn't happened, or loved seeing that did happen! Here are the main crossovers I'd love to see:

Batman (the modern animated version) battles The Shredder (the modern animated version), and meets the TMNT (modern animated version). It would be cool to see Batman and the TMNT fight, thinking each was the others' enemy, then in the middle of the fight figure out they were friends. But the Batman-Shredder battle would only be fun to watch if it was one-on-one!

I'd love to see an older Indiana Jones meet a young James Bond in the 1950's. They could both be on the run from the Soviets, and cross paths on different missions. (I say an 'older' Jones because he was in his prime in the 1930's; and a 'younger' Bond because he was in his prime in the late 1960's/early 1970's. Any farther apart than the 1950's and they'd be too far from their prime stages in life to make the meeting fun! )

I'd also love to see the Justice League (modern animated version) fight the X-Men (well, the movies version, in animated form). It doesn't matter who wins... heck, it can be a draw! But the fight would be fun to watch!

How about you?

hmmm the master chief versus neo...