Silly title but they are in the most part. It might have something to do with the sunshine of which there is plenty including over Winter. The sunlight hits the pituitary gland sending endorphins round your body to stimulate happy emotions, or something like that anyway. Anyway the average Italian is a happy bunny and always seems cheerful.
Living outside as much as they do seeing people they know all over the place must also play it's part. Italians don't move as much as some nationalities and if they do they tend to stay in the same area so they are generally near family and friends they grew up with.
Instead of sitting indoors watching TV they go out to meetup with them or just stroll round and take in the sights around them. Scientific studies say this type of behaviour keeps people calmer and more grounded. Electronic devices, although on the increase, are much less used, it's more about the closeness factor to other people than talking to them remotely.
Slow living in general plays it's part, nothing too hurried or rushed meaning less stress. I do think that taking time out to just enjoy life can help, sometimes there's just too much to do but if necessary schedule time to play a game of cards with friends. Do you do this already, does it help to make things happier for you?