The parts of the brain shown in red make up the limbic system. Notice it is deep within the brain and is highly interconnected with the outer layer. These deep layers appear to be the brain's pleasure center, which plays a role in sexual arousal and the "high" derived from certain recreational drugs.
Later in our evolutionary development the human brain added more layers on top of the old brain for processing higher level thought processes like language, creativity and reasoning. This "new brain" is called the cerebral cortex and at the right it is indicated in green. The cerebral cortex receives and processes the sensory information from the many different sensory organs such as the eyes, ears, taste buds, nose and touch sensors. Here is where we learn how to function in our modern world as kind, caring, socialized citizens in our particular culture.
It is important to understand that we operate from both the old and new brain at the same time. For example, a healthy young man may be using his new brain for selecting the correct fork to eat his salad at a dinner table while at the same time his old brain is primarily interested in surviving and getting some sex. As he eats his salad and discusses politely with others, his old brain is going to be constantly on the lookout for any woman who might be open to a walk in the garden or quick roll in the hay. It is his new brain with its powerful ability to learn and direct his conscious behavior that keeps him operating somewhat normal and not approaching the hostess for sex right there under the table.
However in this case, his sexual attraction to the hostess will show up unconsciously in his body language. For example, his eyes will not miss a centimeter of her soft bare skin down her collar when she leans over to serve him. His every move will be unconsciously affected by his old brain limbic system trying to signal her he desires her and is waiting for her to respond. When he sits, he will sit in the open-leg position displaying his "goodies". His arms will be wide open and in her direction much of the time. He will watch her every move with great pleasure watching for a positive body language reply to his body language. And this is all done unconsciously!
The fascinating and wonderful part of this scene is that her body language may unconsciously be responding to his body language with affirmative responses to his every move - if not for a real encounter, at least for the fun of watching him try! It will be no accident when she repeatedly leans over to offer him more to eat. She is not just being a good hostess interested in his caloric intake! In the mean time the discussion may drone on about what a wonderful meal it was and how the duck was prepared with orange sauce and etc. That is the socialized new brain talking and doing what it learned to do in this modern world.
The point here is that there are conflicting messages we must deal with in this flirting, dating and mating process. It helps to understand that at all times everyone's body language is accurately and honestly reflecting how they are feeling from moment to moment. 
If you learn to observe and understand body language you will greatly improve all your relationships with everyone you deal with daily. You can choose to respond consciously with your own most appropriate responses that will facilitate your relationship. You may want to curb your own inappropriate body language and therefore present a much better image. Even if others are not consciously aware of reading body language, their unconscious old brain will still understand your nonverbal cues that you send.
If you want to really understand people in your daily relationships, study their ongoing body language.
