The presence of a robot in our everyday life can generate both positive and negative effects on us. While performing a difficult task, the presence of a robot can generate a negative effect on the performance and it can also increase the stress and anxiety levels. In order to minimize these undesired effects, we propose the use of user's motivation , based on the Regulatory Focus Theory. We analyze the effects of using Regulatory oriented strategies in a robot speech, when giving a person the instructions of how to perform a Stroop Test. We found evidence that matching the Chronic Regulatory state of the participants with the Regulatory oriented strategy of the robot improves the user's performance, and a mismatch leads to an increase of cognitive load and stress in the participants.