I suppose it was always valuable, but what made it astronomically value was that someone stole it and then lots of people talked about it - kind of interesting to think about how things like that come to affect the value of an item.
The author was spending time at the beach with family and friends, kids playing on the beach. There were enough and more toys for all kids to play with.
One little girl however spent an inordinate amount of time showering love and attention over one specific doll. And after that all other kids wanted to play with THAT doll only, ignoring all other toys.
Guess that is a commentary on human nature at some level.
In this case, if the item is worth stealing, then surely it must be very valuable to someone, hence it should be valuable to everyone. This being a unique item, i.e. scarcity, pushes its value to astronomical heights. There's theoretically no limit to what someone is willing to pay or do to obtain such a piece. We see this all the time in the art world.
With multiple art pieces being stolen, it's also partially a matter of luck and publicity that Mona Lisa was so singled out to become a legend.
Just saw it at the Louvre myself in June. It was wild how there was that somewhat long line w/ people jockeying for position at the front to take selfies with it.