To some extent, this is a question to which the answers are necessarily quite subjective. However, as a general guideline, I would suggest that you look at the bread as just another ingredient in the dish -- not as somehow distinct from the filling choices. Like any other ingredient pairing, you can evaluate the characteristics and choose things to complement each other. So if you pick your bread first, evaluate if it's rich or mild, savoury or sweet, dense or light, crusty or soft, thin or thick, etc, and then choose fillings to complement those properties.
For your fillings, there's not only the flavour pairing of the filling and the bread to consider, but also the texture/properties of the filling. A dry filling might not be as pleasant on a dense bread because the texture would be too much the same. Similarly, a saucy filling needs an appropriate bread selection to contain it and make it easy to eat.
Many pairings that are considered standard will just taste "good" to us because of tradition and familiarity, but originally, someone probably put them together for a reason. That doesn't mean it's not worth trying something different, though. With the globalization of cuisine, we have many more filling/bread choices available and you might find a new favourite by pairing a bread from one culture with a filling from another, based on their complementary qualities.