It is traditional to place a mezuzah on every door in the home (excluding bathrooms). It is also traditional to ritually kiss the mezuzah as we pass it by touching it with our fingers and then bringing our fingers to our lips. This reflects an awareness of the transition involved in leaving home and entering the outside world, and in leaving the outside world to return home. Home provides us with a sense of familiarity, safety, and family, of "ourness." On the other hand, the outside world is a place of opportunity, challenge, unclear boundaries, even of risk and danger, populated with people known and unknown. Kissing the mezuzah marks that transition at the boundary line of our door.
Our kavanah then is to be aware of the movement from home to world, a journey fraught with dangers real and imagined. It is a journey that promises opportunities for growth and success, along with opportunities for bringing holiness to all our interactions.
— Michael Strassfeld in A Book of Life