Many people touch the mezuzah and kiss it when they pass through door. The source of this practice is a story in the Talmud (Avodah Zarah 11a) about the famous convert and biblical translator Onkelos. His uncle, the Roman emperor, peeved by Onkeles’s conversion, sent three different delegations of soldiers to return his nephew to Rome. But each time, Onkelos converted them to Judaism.
With the final group, Onekelos pointed out a mezuzah, touched it and explained that whereas most kings dwell inside castle walls and have soldiers protect them, God, the King of Kings, dwells on the doorposts and protects the people inside. The Romans were won over and they converted also.
This inspired many rabbis to rule that we should do so every time we pass one. The great kabbalist Rabbi Isaac Luria recommended kissing the mezuzah too. Maimonides said we should “be careful with the mezuzah because it is a constant obligation on all of us.” He was clear that the mezuzah is to promote divine awareness but is not some kind of talisman and he did not recommend kissing it.