Following the events of Deckard's abduction in Blade Runner 2049, K meets the revolution, then is sent home. However, he is seen walking somewhat forlornly, apparently lost in thought, making his decision.
Then the Joi advertisement reacts to him, calling him "lonely" and "a good Joe". He then flashes back, and pulls out Deckard's gun, apparently having had made his decision.
I'm slightly confused as to what it was about this interaction that caused him to trigger. The memories talk about dying for a cause, and freeing themselves from slavery, and the advertisement, to me, seems like the opposite of that - dehumanising his companion by simply making her a product.
What was it about this interaction with the Joi ad that triggered K into action?