Why we talk about "love" & work to create "common knowledge"?
The short version is that "common knowledge" massively boosts cooperation and collaboration. 

And in The Descent of Man, Charles Darwin mentions “love” 95 times & “survival of the fittest” only twice. “In the long history of humankind (and animal kind, too) those who learned to collaborate & improvise most effectively have prevailed.”

Here's the longer explanation:

“What we found was that, for private knowledge, even if we varied the payouts, or the number of people involved, only about 15 percent of people cooperated,” Thomas said. “With shared knowledge, we saw about 50 percent, and with common knowledge, it was 85 percent. It was just a whopping effect. That indicated to us that we are very sensitive to this previously unappreciated mental state. Our minds evolved to understand this important kind of social structure, and how different kinds of knowledge can impact it.”

The effects of common knowledge are hardly limited to the type of economic games described in the study.

“You can see evidence of these coordination problems everywhere. We’ve done work on euphemism and indirect speech, where everyone understands the subtext of what’s being said, though it isn’t explicit. You can also see aspects of it when people talk about taboos or political correctness. When something is taboo, that’s a common-knowledge issue because even though everyone may think it, you can’t say it. There’s even evidence that self-conscious emotions, like guilt or pride or shame, are sensitive to common knowledge, and that certain emotional signals like blushing or crying are built around the idea.”

Thomas KA, DeScioli P, Haque OS, Pinker S. 2014. The psychology of coordination and common knowledge. J Pers Soc Psychol. doi: 10.1037/a0037037.

Abstract

Research on human cooperation has concentrated on the puzzle of altruism, in which 1 actor incurs a cost to benefit another, and the psychology of reciprocity, which evolved to solve this problem. We examine the complementary puzzle of mutualism, in which actors can benefit each other simultaneously, and the psychology of coordination, which ensures such benefits. Coordination is facilitated by common knowledge: the recursive belief state in which A knows X, B knows X, A knows that B knows X, B knows that A knows X, ad infinitum. We test whether people are sensitive to common knowledge when deciding whether to engage in risky coordination. Participants decided between working alone for a certain profit and working together for a potentially higher profit that they would receive only if their partner made the same choice. Results showed that more participants attempted risky coordination when they and their prospective partner had common knowledge of the payoffs (broadcast over a loudspeaker) than when they had only shared knowledge (conveyed to both by a messenger) or private knowledge (revealed to each partner separately). These results support the hypothesis that people represent common knowledge as a distinct cognitive category that licenses them to coordinate with others for mutual gain. We discuss how this hypothesis can provide a unified explanation for diverse phenomena in human social life, including recursive mentalizing, performative speech acts, public protests, hypocrisy, and self-conscious emotional expressions.