I think it's because people tend to be afraid or anxious of people and situations that are less familiar to them. It is easier to decide that you won't bother to try to get to know someone who is different from you, because you can choose to do nothing. It is much harder to make an effort to understand someone who is different from you than it is to do nothing at all.
I think that talking about the fact that people tend to be more alike than different helps people to be more accepting of others who are different. Most people want the same things in life: to feel some measure of success, to feel included and to feel loved.
People are used to and prefer people more like their self, therefore they will be slightly hesitant to talk to someone different because they have not spoken to many people like them before.
In order to get people to talk to people different than them, the person they are talking to may introduce themselves by informing them about him/her self.
