I was thinking about it a few days back, and though its a question of sentiments, emotions and feelings, I believe there is still a mathematical relation to it. The equations remain to be invented, but the basic theology goes like this. Just like sherlock holmes once described the mind as an attic having finite ability to store furniture (information, as is the case with the mind) ... so is the fact that the result of an action is influenced by the sum of two variables - what the mind thinks, and what the heart feels.
The mind is something which contains intelligence. Its rationality, practicality and intelligence clumped into one small place. All those three factors affect each other and hence the final output from that part of the body. The mind will always direct the body to do whatever is the more sensible thing, and whatever is the right thing. It will never make us do that might result in personal harm, pain ... or even something that might affect the person negatively. Hence, the most pragmatic approach!
The heart, is the opposite. It'll bid to do whatever causes the least amount of pain to others, and one it loves. No matter the pain, hurt or thought involved. The righteousness or rationalities are forgotten, or pushed aside. If something can be done which will rid someone it loves off pain and discomfort, it will not think twice before pushing the body to go ahead with it. All it wants is the person it loves to be happy.
Now, the final action is the sum of these two variables, and which of the two has more influence (higher value). The ideal situation, which surfaces in very few cases, is when both the variables share an equal magnitude and hence have an equal influence. In this case, the step taken is usually the only one possible. However, as in most cases, if any of the two takes a higher value, its tendencies will dominate the final action taken, and hence the consequences will be seen. Now, how do we know which one will have the higher influence? That, my friends, depends from people to people. Ones who are akin to thinking and acting, will have a more developed response to reality, and hence think hard and well before they take any step. But the ones who feel, and are sensitive to things, will usually have a higher value for the 'heart' variable, and hence their actions will be to first comfort their loved ones, and then think about themselves.
But, in the end, man is a very complicated animal, and hence there many sub-variables to these two variables which have to be taken under consideration. To list them here will take long since they are many, including the proximity to the people involved in a situation, how the outcome might affect them, how the outcome might affect the do-er, and so on. Hence I'm not going into that aspect of it. With this knowledge, maybe it'll be easier for you to predict what yours and others behaviour and reaction to certain situations might be. Again, I might be wrong with my theory, but hopefully I'm not. Any arguments (as is usual with the theories) are more than welcome!