Note that not all people can taste "bitter". I know of a rather common genetic mutation in the TAS2R38 gene that allows people to be able to taste certain "bitter" chemicals that others cannot. People who have no copies of this mutation are nontasters. People who have 2 copies are able to taste chemicals, like PTC, very strongly. And then there are people who are hetertozygous (have one taster copy and one nontatser copy) that are able to taste these chemicals to a certain extent.
To sum it up, people might not be able to taste the bitter in their foods to the same extent, or at all, as others. This could very easily explain why someone might like bitter foods.
in addition, it might be all psychological. at this point in my response i have no facts but only theory. It could just be a personality thing. I have friends who like to drink black coffee and incredibly sour candies. Simply because they can. It started as a sort of satisfaction that they could not be bothered too much or could control their reaction to the taste. What started as bets among friends as to who could handle a cry baby better eventually turned into an acquired appreciation for other bitter and sour foods.
hope this helps!