This is really not such a silly question PC. We usually associate the word 'enemy' with something not pleasurable and therefore we tend to dismiss the thought right out of our minds without looking at what the enemy can teach us about ourselves spiritually.
Essentially, the 'enemy' is the embodiment of everything we find disagreeable. The 'enemy' is the personification of our own bias, prejudice, our fears, the opposite of our willful opinions, the crack in our self righteousness and the thorn in our comfort balloon. Having the enemy gives us the opportunity to ask what it is that made the person our enemy. It is a chance not to focus on the enemy's actions but our reaction towards the enemy's actions.
For example, if the enemy slandered us and we feel deeply aggrieved, it is an opportunity to examine if we are very attached to a certain kind of reputation that we would prefer to have. When we closely examine it, the reason I am aggrieved is not really the enemy's slander...there are after all, empty words, but my own hurt feelings as a result of being denied my strong attachment to having a good reputation. And we become unhappy because we always associate happiness with our attachments. Fundamentally it is our attachments. A 'Friend' is our attachment to Happiness and things we Like and an 'Enemy' is our attachment to Anger.
Although it is a lot easier said than done, an enemy is the beginning of the training to combat the Eight Worldly concerns. And if we are serious about our own spiritual growth, then the enemy becomes not the object of our practice but the trigger to remind ourselves of that practice.
As we all know, the true enemy is a mind that shouts: I want, I like this and don't like this, this ticks me off whilst this other thing makes me happy, I want more, why can't I have more and so on.