Why are Miles and Jack even friends?

I am curious as to why Miles and Jack would have remained close friends since leaving college. They are completely different characters that don't really understand each other and have nothing in common. Their massively different goals in life mean that for most of their trip they are working against each other's aims. Is there some sort of playful dynamic that these particular sorts of opposite characters have that, in real life, often works? Or is it simply a case of co-dependence?