The Left Side
The mezuzah belongs on the right which represents the positive, dominant aspect of things, specifically the right-hand side as we enter. The Zohar points out that as a result, as we leave the room the mezuzah is on our left - seemingly focusing our attention on the absence of holiness! Here is one understanding of this seeming paradox: As we enter our homes, where we have the ability to create a positive atmosphere of holiness, the mezuzah is on the positive side, the right, reminding us to "do good". As we move towards the street, where evil has sway, our main challenge is to "turn from evil". The mezuzah is found on our left side, where it can confront the negative challenges we face.
(Rabbi Asher Meir)