Weather or not

This morning a colleague commented on the fact that the sun wasn't shining. I thought it was a perfectly lovely morning - calm, warm and dry.  Anyway, it set me thinking that for most people the weather dictates whether they will be happy that day or not.  Blue skies and sunshine and everyone is happy but on the cloudy days a smile seems hard to find.  Why is that?  Why should we let the weather decide if our day will be good or bad, whether we'll smile or frown?  Yes, life has its ups and downs and there are some days when dark clouds are just an added misery.  But whether we're happy or sad, grumpy or mad, what's the weather got to do with it?