Many people touch the mezuzah and kiss it when they pass through door. The source of this practice is a tale in the Talmud (Avodah Zarah 11a) about the popular Jewish convert and also scriptural translator Onkelos. His uncle, the Roman emperor, peeved by Onkeles’s conversion, sent 3 different delegations of soldiers to return his nephew to Rome. Yet each time, Onkelos transformed them to Judaism.
With the last group, Onekelos pointed out a mezuzah, touched it and described that whereas most kings stay inside castle walls and have soldiers protect them, God, the King of Kings, makes His home on the doorposts and safeguards the people inside. The Romans were converted as well, being won over by his words.
This inspired lots of rabbis to rule that we ought to do so every single time we pass one at the door. The excellent kabbalist Rabbi Isaac Luria recommended kissing the mezuzah too. Maimonides claimed we must “be careful with the mezuzah since it is a constant commitment on us all.” He was clear that the mezuzah is to provide magnificent understanding however is not some type of amulet and he did not suggest kissing it.