All the same?

This definitely isn't the first time this is happened to me, but I was recently in a psychology class where we started discussing cultural differences. Someone in the class mentioned that they didn't think there were any differences and they just viewed people as people. Everyone is the same. This person proposed that people have similar fundamental values and core beliefs. I have definitely heard arguments like this before and it got me to thinking, is assuming that we all have the same fundamental beliefs what gets us into trouble in the first place. I think even within cultures we cannot assume that we're the same. Suppose a girl comes from a family where fidelity is valued and the guy she starts dating doesn't believe it is until rings are exchanged. They never voice their views because they assume that the other person has similar values. Are we really all the same?