Why did Athens care so much about Sicily?

It seems like Athens was pouring resources into a war in Sicily that was far from a guaranteed win.  (I believe Thucydides said they sent 80 ships to it.)  At first glance this just seemed stupid to me.  Why, when you are already fighting a multiple front war and you know that you may have to start fighting on several more fronts very soon, would you add another front.  The humanitarian cause of helping one of your colonies that isn't even that integrated into your empire might make sense in peace time, but it clearly doesn't make sense when you fighting Corinth and other states in main land Greece.  The idea of expanding the empire was the only thing that made sense to me, but there are much better times to expand your empire especially when you have to maintain a long sea based supply line passing by waters right by states you are warring against.  This still didn't make sense to me, then I remembered Rome's servile wars.  The first two were both in Sicily.  They were put down fairly easily, but I'm pretty sure they were important for 2 reasons: 1) Rome doesn't want the slave revolt expanding as it ended up doing in the 3rd servile war and 2) I'm pretty sure Sicily was a major grain producer.  Rome (the city and empire) was essentially dependent upon Sicily and Northern Africa (including Egypt) for its massive grain consumption.  I think that Athens wanted to secure a grain supply that was independent of mainland Greece and wasn't going to be easily tampered with by the Persians who controlled Egypt and could have cut off any grain supply from the black sea region by conquering the land on both sides of either of straits going into the black sea.  The only remaining grain sources were Sicily and the land around Carthage.  It couldn't take Carthage and Carthage could probably charge through the roof for its grain when trading with Athens.  That just left Sicily.  That is the only reason I can think of for why they devote so much of its resources to fighting in Sicily when they weren't entirely secure in mainland Greece.