I think that the Mona Lisa painting is quite unique. There are many different aspects of the painting and the smile I do find most intriguing. The smile is very different in many ways. I think the painting is not a big mysterious painting at all. There are many logical explanations for it that many come up with but, of course, I do believe that the smile simply reflects what Leonardo Da Vinci was feeling or simply what he likes to see.
People often take a simple thing and turn it into a big debate.
It is a theory that people create things the by the way they were feeling. If someone is sad then they paint or draw or sketch something sad. So, I don't know how many people will agree with me on this one, but hopefully they will. Any painting reflects the mood of the creator and in this case he was feeling happy. Maybe an illegitimate one, but no doubt his painting must have some hidden reference to this daughter.