I agree with this - it's the human connection that does it for me. As someone else already mentioned, seeing the brush strokes on the canvas and knowing that the physical thing in front of me was crafted by the hands of the artist however many decades or centuries ago, is a special experience.

Interestingly, when paintings like the Mona Lisa are concealed in those plastic protective containers, that experience is significantly decreased for me. It's strange, but that layer between the canvas and my eyeball has a major effect on my perception.