These lines capture a feeling I get occasionally, and I am sure everyone gets. Next time you are in this mood you can remember these lines; I like particularly the way he is in a good mood for no reason, just sitting down somewhere, and he specifies the short time the mood lasts for (the word "blessed" should be pronounced with two syllables):
My fiftieth year had come and gone,
I sat, a solitary man,
In a crowded London shop,
An open book and empty cup
On the marble table top.
While on the shop and street I gazed
My body of a sudden blazed;
And twenty minutes more or less
It seemed, so great my happiness,
That I was blessed and could bless.