Blessed are the merciful, for they will be shown mercy.
It is quite easy to show mercy externally and to lack a truly merciful heart. A truly merciful person is not quick to judge the righteousness of another. A merciful person doesn't wonder whether the person she shows mercy to "deserves" mercy or sympathy. A merciful person is aware that she herself needs mercy.
Some people like thinking of themselves as perfect; they don't believe they need mercy. Because they have managed to attain what they consider to be perfect spirituality, they allow themselves to judge others. They are like the pious pharisee who tells God all that is good about themselves and compare themselves to others...always making themselves superior in their own eyes.