Right now, you are experiencing the same level of mental stimulation you would experience if I had just given you £16,000, according to some research funded by HP.
Whilst this kind of comparison makes no sense to me at all [and to my mind demonstrates just how little we can really understand from seeing blips on the fMRI], it's still a pleasant reminder of the power of positive interaction and the fact that seeing other people happy will make you happy. Probably something to do with mirror neurons and almost certainly a herd idea.
Additionally, as Mark mentions in Herd, people can always see through fake smiles. So this leads to the question - is it enough to show people smiling babies, or do you actually want to make people smile? Do the smiles in ads seem real?
As the article points out, the high street retail environment has a huge potential advantage over online retail in this respect - brands have the opportunities to smile at their customers, face to face.
Of course, since we can see through fakes, this requires having employees who are actually happy.
Regardless, it's certainly worth remembering that your employees are part of the herd too.
[Incidentally, it's also the case that if you force yourself to smile properly, this will in turn make you feel happier - the physical act is thought to trigger emotional response.]