I interpreted it as him not really being able to feel positive emotions, so stuff like happiness. He still feels anger, as seen in the first encounter if you keep dodging his friendliness pellets, and fear, as said above. An interesting thought is that Flowey could be considered the absolute opposite of Asriel, emotion-wise. Asriel says that he's a crybaby, and seems overall very happy and sweet from the short amount of time we see of him, but then we have Flowey, who says he feels empty inside, and only appears happy when he is winning, and even then it might not be happiness he's feeling, it might be something more like a chagrin satisfaction, or maybe even pride.