as a sign of respect

What does Prophet Muhammad (peace be upon him) teach us about being merciful to others?
 Narrated Abu Huraira:                     Allah's Apostle kissed Al-Hasan bin `Ali while Al-Aqra' bin H`Abis at-Tamim was sitting beside him. Al-Aqra said, "I have ten children and I have never kissed anyone of them," Allah's Apostle cast a look at him and said, "Whoever is not merciful to others will not be treated mercifully."