Just 200 years ago, there wasn't a strict ethnic division between Serbs and Bulgarians. There were two transitional ethnicities Shops and Torlaks. They lived in what is nowdays eastern Serbia, western Bulgaria and northern Macedonia. They spoke language that was a gradual shift between modern scholar serbian and bulgarian. Torlak dialect a little bit more resembled serbian, and shop bulgarian. When borders were finally drone between 3 countries (1913), those who ended up in SRB eventually became Serbs and vice versa.
For Serbs, if they can make Bulgarians speak more slowly, they understand them pretty well. The same goes around, with Bulgarians having a easier task as Serbs do tend to speak a little slower.