Why do Presidential Candidates frequently choose Vice Presidential candidates that are opposite or very different than them?

After graduating from college with a degree in education, matt found a teaching jobin saudi arabia. he figured that a teaching job in the middle east would be the same as a teaching job in the united states. while there were similarities between american and saudi schools, matt was unprepared for the differences between the two cultures. on the first day, a number of his students arrived later than the class's designated 10 a.m. starting time. later, during a break, he noted that his students seemed to stand closely to each other and to him when they spoke. after class, a confused matt asked the american director of the teaching program about these observations. that's why it's always important to do your homework! the director responded with a laugh. what is the possible explanation that director could gave him?