In a word? Novelty.
I talk to myself every day – and I pretty much know, based on years of such interaction, what I'm likely to say back. Now and then, I'll surprise myself, and feel an immense creative spark, or "unsticking" of a problem… But 9 times out of 10, that process is much faster if the majority of the talking is done with other humans, who carry different life histories, perspectives, and conversational styles.
It's the newness, the surprise, and the unexpected delight (or, sure, sometimes horror) of interpersonal interaction that makes them so powerful a force in our lives.