Common points of reference to share early in a relationship can be job/family/humour and are based on the situation in which folks find themselves meeting. e.g. I'm not going to tell you about my theatre history if we've just met at an IT meeting. (not without a g&t anyway)
There's nothing wrong with figuring out why you do/don't like someone through what you first discover you have in common. That's human nature. We want to work/play with people we like.
Where it can fall apart is when you don't explore further.
Some folks just want to know how you're like them. They don't want to look beyond their comfortable parameters. Forming a community/team/circle with people who think the same way about most of the same things is a common practice.
Others form a bond based on the common points of reference and then deepen the relationship by exploring the uncommon. These communities/teams/circles may have folks with a common philosophy but very different approaches/answers/ideas.
Without challenges, alternate viewpoints and the age-old question: why?, we would not see change.
Yes, make sure you like/respect each other before uncovering the differences. But don't stop at the similarities. I don't admire folks because they are like me; I admire those who inspire me to do things better/differently.
By all means, enjoy your mentor/mentoree but isn't the point to push each other into new territory?
Which prompted me to tweet on Saturday: Do you prefer to follow a person or a cause?