Thinking people have always sought a right reason for people to feel self-motivated to love each other (which is the goal of all religions). In the process each religious leader presented some reasons which were not adequate because division appeared once the founder left the stage.
However, Carl Sagan (astronomer, astrophysicist, and science-fiction novelist ….etc) got it right when he wrote: “For small creatures, such as we, the vastness is bearable only through love” (Contact, Chapter 24 p. 430). When an interviewer said this was the best of his quotes and asked him to put it in simple words, Sagan said: “We exist in vastness of the universe, hence should have vast (great) mind of treating every one as residing/subsisting on the same (planet) home who breathe the same air (in contrast to going into smallness of contraction with each one seeking own interest). Where ever we look, we hear “be of vast/great mind, and there is no way we can escape this eternal command.”