Wisdom from the Fathers - St Maximos the Confessor

One can distinguish five reasons why God allows the devil to attack us: 

First, so that from attack and counter-attack we may become practiced in discerning good from evil; 
Second, so that our virtue may be maintained in the heat of the struggle and so be confirmed in an impregnable position; 
Third, so that as we advance in virtue we may avoid presumption and learn humility; 
Fourth, to inspire in us an unreserved hatred for evil through the experience we thus have of it; 
Fifth, and above all, that we may attain  inner freedom and remain convinced both of our own weakness, and of the strength of Him who has come to our aid.