To understand your boss you should understand his/her likes and dislikes. To know this you need to build good rapport with them, ask them open ended questions and let them speak more..When you build good rapport they automatically will start trusting you. This will enable you to get close with them. Once you have built rapport start asking open ended questions to your boss. For example:- How do you handle the job with a calm mind?, What are the biggest challenges you had to face in your career?, What is the goal you have for our company and yourself?. All these questions will make him/her open up. Listen very carefully to their language pattern, body language, tone and pitch.