There are times when the heart is in bad shape. Bad mental qualities get mixed up with it, making it even worse, making us suffer both in body and mind. These bad mental qualities are said to be “unskillful” (akusala). The Buddha teaches us to study these qualities so that we can abandon them.
There are other times when the heart is in good shape: at ease with a sense of well being. We feel at ease whether we’re sitting or lying down, whether we’re alone or associating with our friends and relatives. When the heart gains a sense of ease in this way, it’s said to be staying with the Dhamma. In other words, skillful (kusala) mental qualities have appeared in the heart. The skillful heart is what gives us happiness. This is why the Buddha taught us to develop these skillful qualities, to give rise to them within ourselves.
– Ajaan Suwat, Thai Forest Tradition.