Even a casual follower of the Irwin's family's adventures is well aware of how large the late Crocodile Hunter looms in their hearts.
And while part of that obviously has to do with the fact that they're surrounded by his legacy every day and carrying on his life's work, it's clear from the way they talk about him that this would be true even if they all took different paths.
So when a milestone comes up that he's not around to celebrate, it's hard to let it pass without thinking about him. And that's especially true today, as it would have been his 60th birthday if things had turned out differently.