Since the neural network of the brain is inside our heads, it doesn't come as a surprise that we associate our minds with our heads. But why do we associate emotions with the heart?
The physical heart is pump, keeping the blood flowing in our bodies. The psychic heart also keeps 'the psychic beat' and regulates the flow of psychic energies. The heart center (or heart chakra) is also in a central location considering the psychic energy system of our bodies, and performs an important act of balancing.
With an open heart, the psychic energies flow freely and in a balanced way. Through the psychic heart we can feel the connection to the world around us - through our hearts we can feel Love. Love is the oneness of the whole flow, not so much the giving and taking of psychic energy which are only aspects of it. If you are thinking about giving love and taking love, then it's more just an exchange of psychic energies rather than expanding your presence and identity to embrace all that you Love.
How to have an open psychic heart? Love is a form of death: the closed ego dies and the things you Love become the new you. Death in this sense is more like accepting change and also expanding the sense of time - your future self is also a part of you NOW.
Being fearful, unallowing or judgmental means that you will perceive the more intense flow of energies through your heart as threatening and destructive and will reflexively close your heart as soon as it opens.