Your favourite subject is out there, but there may well be more than one way of approaching it if it is offered as part of a joint course. This isn't a compromise between two subjects, but a bold decision to learn something new and discover the way your subject relates to something else. Modern scholars are increasingly finding themselves working over disciplinary boundaries, and joint courses are one way in which we keep up with this trend.
Philosophy, Politics and Economics, for example, is one of Oxford's most famous courses, teaching students how to understand the creation and development of societies in three different ways.
Maths and Philosophy is founded on the belief that the parallel study of the two related disciplines can significantly enhance your understanding of each, finding common ground in logic and analytical thinking.
Psychology, Philosophy and Linguistics takes this approach even further, taking subjects which on first glance are not immediately related and giving a chance to find their surprising intersections.