Seeing bluebirds make me think of happiness. This of course makes me happy. Then I smile, and that reinforces the happiness.
A couple years back, a bluebird was never seen here. Now there are bluebirds everywhere, so it’s rather impossible not to be happy — everywhere.
Of course, happiness doesn’t have to be attached to anything (or anyone). It is possible just to be happy, with no reason for it.
But I find bluebirds are a great excuse.