They say opposites attract, and eldest children often marry youngest children. I agree that opposites do attract, but I think you still have to have some core things in common.
For instance both Mr. Bee and I are INTJ's on the Meyers-Briggs personality test (which is a rare combo). But we're also very different in that I love adventure/travel/change whereas he is the complete opposite.
But we have similar views on religion, education, and we're both introverts. I think if one of us were an extrovert and the other an introvert, that could be really hard!
Are you and your SO opposite?