Happy... but why?

The eve of Independence Day was one of those days when I was all hale and hearty and was ready to try out anything that came my way. I don’t really know why I was in such a mood. I was just too happy (I think) about something. It was like one of those moods when I felt I had nothing to worry about, I had no problems in life and all my dreams (I don’t know which dreams too :P) were coming true. Du was surprised at my mood. I can't blame him... coz I was surprised too.
I don’t know if you have ever experienced such a thing but it feels really good to be in one of those moods.
Now someone help me out with this... Is there always a reason for a mood a person is in?
And if there is... then I am lost as usual :)