That's because beef liver has a lot of phenylalanine in it and those who have the genetic predispostion to taste phenylalanine it is very, very bitter. I have that genetic trait. I would assume you probably do too.
But I like it too. I did my share of damage to that platter of liver and onions. I don't indulge often though ... once or twice a year. Bad tickers run in my family.