Why is the liver so good at regeneration?

It's true. You can remove up to 75% of the organ, and it will regrow!

Once we reach adult form, our organs vary in their ability to regenerate, but in general they largely don't. The exceptions to this rule are instructive. Skin regenerates well, and the reason is obvious: it is exposed to constant wear-and-tear from brushes with the environment. The endothelium lining the gut also regenerates quickly, and again this is probably to cope with the wear-and-tear of food in transit.

And the liver? Well, its regenerative abilities might be for a similar reason. The liver is the chief 'detoxifier' of the body. Via the portal venous system of the gut, it is in direct contact with any toxins absorbed from our food, and via the hepatic arterial system toxins from the rest of the body hit the liver. The liver has a formidable array of chemicals to neutralise this threat, but inevitably it must frequently be damaged while doing so. And it is believed that its remarkable regenerative abilities are an adaptation to deal with such damage - the liver can recover completely even if the majority of it is destroyed.

Why don't all organs have this ability, though? We'll cover that in another post...