Many of us have a tendency to see people through rose-colored glasses rather than judging them for who they really are. In other words, we're idealistic rather than realistic. We know what people are capable of -- they've done us wrong before -- yet still we decide to give them the benefit of the doubt. We really exclusively on what our heart tells us, to the exclusion of any alarms your mind may be sounding. Unfortunately, this is how people end up in disastrous relationships.
When dealing with people, it's imperative that we not solely rely on our emotions. The brain should also play a role.
We can compare this to something in politics of all things: the system of checks and balances, where each of the three branches of government can limit the powers of the others. This way, no one branch becomes too powerful. Each branch “checks” the power of the other branches to make sure that the power is balanced between them. It's precisely because of this system that the president and Congress -- and even both houses of Congress themselves -- have so much trouble seeing eye to eye.
Similarly, the mind serves to keep the heart in check, and vice versa. We should never allow emotions to cloud our judgment, nor should we try to rationalize our way out of every relationship opportunity that comes our way.
I'm not saying we should be cynical in our relationships and expect that people will hurt us. However, when someone has already let you down, you must keep in mind that he or she is capable of doing it again. Don't be fooled twice.