Opposites attract?

I have a long-time friend. A friend that continues to be probably the best friend I've ever had. Ours is a low-maintenance, relaxed friendship. We can talk everyday, or go for weeks without communication and still, the friendship remains strong. We also happen to have kids the same age, and therein lies the problem:

Why don't our kids get along?

In all honesty, they're doing much better (maybe it was the peace offering in the form of candy we brought last time we got together). But there was a year or two there that they spent as much time fighting as they did playing together. I know, age-typical behavior. But we always thought our children would be instant best buddies, too. My real concern falls 20 years down the road:

How are these two darling, strong-willed kids going to fall in love and marry someday if they don't even like each other?


I guess we can only hope that opposites really do attract.