Unlikely Allies

The only logical explanation I could think of for two people with different backgrounds and interest becoming unlikely allies, would be to take down a common enemy. I mean, why else would opposites attract? That's like spider man teaming up with venom or superman teaming with Lex Luther. The only time people with such differences become allies is to complete or reach a common goal. I know I wouldn't team up with my enemy unless it was for that reason. I guess that's why they say keep your friends close and your enemies closer.