It's quite simple, really.
The matter in our universe is comprised of the same elemental stuff throughout. Proportions differ and it might be mixed together differently here and there, but it's the same stuff. Some of those mixes develop self-replication under certain conditions. Self-replication can perpetuated through the ability to use energy, respond to external stimuli, grow, reproduce, etc. and we call that life. Different forms of external stimuli have favored sophisticated sensory systems, complex nervous systems, memory, and communications in certain branchlets of a vast evolutionary bush.
One tiny bud on a branchlet of that bush has developed in an extraordinary way. Tremendous brain power – perhaps first developed so well to help make sense of complex social dynamics in a long-lived species, to support spatial memory of resource allocation during lean times, and to promote complex coordination of action and anticipation of events that enhanced survivorship – happened to combine high intelligence and manual dexterity. Those were the raw materials for technological advances that created abundance sufficient to afford the luxury of devoting our intelligence to matters beyond mere survival. One result of that was the desire and ability to study, catalog, and develop tests to help us learn more about the natural world. From within those endeavors came the revelation that... we are comprised of the same elemental stuff as the stars and planets and comets, etc.
WE are the physical and intellectual manifestation of a sentient universe. The universe developed the ability to examine itself. That's us. That's it. Sagan's "We are a way for the Cosmos to know itself" is not only more profound a revelation than any fairy tale in any holy book, it carries the added gravitas that comes from being true.
