Because the liver handles both metabolism and toxic waste you can damage your liver by mishandling both.
Metabolic-wise, obesity is the primary culprit. When you too much sugar/fat more fat is stored in the liver than it can handle leading to a fatty liver. The fat piled up in these cells takes up precious resources eventually leading to cell death.
From a toxic point of you, the primary cause of liver damage is alcohol – even modest amounts of it cause liver damage. Contrary to popular belief, alcohol does not protect against heart disease: early studies shown that it did because the group of people that didn’t drink included both people who never drank and people who stopped because of alcohol-related health problems. Obviously, people who stopped for alcohol related problems diet sooner.
Another important toxic are drugs. I am talking about drugs prescribed by your doctor, that get all the bad rep, but also “natural” remedies, herbs, illegal drugs, smoking, caffeine, oils… There isn’t a single exception to this – not even the drugs that heal liver damage cause some degree of liver work (fortunately they promote regeneration in a much larger scale).
Finally, food is another source of toxic substances. Sugar and fat are the primary causes, like we have seen, but also pesticides, human-added substances, and even toxic waste produced by the food itself, like oxidants and protecting chemicals.