Bitterness is one of the five main tastes we can detect. Coffee always tastes bitter, in a sense, because caffeine tastes bitter. People who don't like coffee (we don't know any such people) often cite bitterness as the reason. But why do some dislike bitterness less than others?
Our dislike of bitter tastes stems from the evolutionary function of taste itself: a protection against dangerous foods. We recognize many toxic substances as bitter, so what causes a negative reaction: don't eat that! Besides caffeine, quinine - originating from the bark of the cinchona tree, and used for tonic water - or naringin - found in grapefruit - are also well-known bitter substances.
But, despite the negative association, bitter tastes can also do good. When carefully combined with other tastes, bitterness can round out a drink. That is why we find bitterness in many of our favorite drinks and foods: Coffee, of course, but also beer, wine, chocolate, and cocktails like a Negroni or a gin and tonic. Bitter and tasty can go together.
How can you make bitterness palatable? The trick lies in balancing the tastes. Our palate distinguishes between the respective tastes in relation to each other. If something tastes only bitter, we perceive it as overwhelming and disgusting. But, if the bitterness is balanced with a suitable sweetness, the two tastes would balance each other out. The bitterness can even refine the sweetness, and vice versa, through the mutual contrast between the tastes.
Unfortunately, it's not all in the cup of coffee, because different people also have different tolerances to bitterness. Researchers have found a genetic link to bitterness sensitivity, which means you may have gotten your love of coffee from your parents. The gene TAS2R affects how many and what kinds of bitterness receptors we have, and the difference in sensitivity can be really huge: some bitter tastes can taste okay to some, and be completely disgusting to others.
Maybe that explains the strange people who don't like coffee...