At one point I attended an event on creativity, where one of the conclusions was - albeit phrased somewhat differently - as stated in the heading.
The reason is simply that you do not get inspired by people who say the same as yourself - mostly people who have the same background as you or in some way resemble you. Inspiration comes from people who are different than you are.
People who agree with you in everything, who are in the same point in life as yourself, are not able inspire you; you can use them to pat yourself on the shoulder, and although that itself may be very nice, it's not what moves the world forward.
What moves the world forward is people to disagree with, or to discuss with, whether or not it originates in constructive criticism or an curious question as to why you behave the way you do.
Even in the company of the people you agree with, you should look at the differences between you and them and examine them.
Because it is thereby you are going to consider whether your attitudes are rational, and to be inspired to do new and exciting things.
(Translated from Livet er for kort til folk som mig, originally published December 27th, 2011)