He couldn't really remember feeling Tadashi being sad or hurt before now, but some part of him put two and two together and realized that both Hiro and Tadashi were very sad. Activating in the corner, Baymax tilted his head and he remarked his usual greeting, "I am Baymax, your personal healthcare companion. I sense that your emotions indicate you are sad, Tadashi. Why are you sad? I do not understand."
Looking at all of them, they all looked sad. He couldn't quite understand how this worked, but he had an understanding that they were both very sad. For some reason. Shouldn't they be happy? Tadashi was alive again. Everything tells Baymax that they should be happy and not sad.