Allah's Messenger (may Allah's peace & blessings be upon him, his family & companions) once kissed his grandson Hasan while a man named al-Aqra ibn Habis al Tamimi was sitting beside him. The man said, 'I have ten children and I have never kissed any of them.' The Prophet (pbuh) looked at him (sadly) and said, 'Whoever does not show mercy will not be shown mercy'. (Bukhari)