Eren didn't engage in much conversation with Reen.
He had mentally prepared himself for what he was about to do, and it was time to take action. Staying with Reen longer than necessary would only stir his emotions, something he aimed to avoid.
Eren surmised that Eliza's intention in allowing him to reunite with Reen was to expose his vulnerabilities, hoping to exploit them. Nevertheless, he took the bait because, for someone like Eliza, glimpsing his vulnerabilities would be the key to making her believe that everything was still under her control.
From one control-freak to another, he understood her psychological needs.
