Bulgaria is a Slav Orthodox nation, like Russia and Serbia. And, like Serbia, it recovered its independence in the 19th century after about five centuries of Ottoman Muslim rule.
Like Serbia, it has a troubled relationship with its own Muslim minority. In spite of that, Sofia's sympathies are with the West rather than Belgrade, although the country's former communist rulers are much keener on Slobodan Milosevic. One reason for Bulgaria's pro-Western tilt is the obvious financial one; Bulgaria is poor, wants EU and Nato membership and won't jeopardise its chances by refusing Nato access to its airspace.
But there is a historic reason, too. In the 19th century Bulgaria and Serbia were fierce rivals in the Balkans and the two countries went to war in 1885 and 1913. They were on opposite sides in the First and Second World Wars.
For decades Bulgarians fiercely resented Serbia's annexation of Macedonia, and in the 1920s and 1930s Sofia was a hotbed of anti-Serbian terrorists, who helped to engineer the assassination of King Aleksandar of Yugoslavia in 1934. Those old grievances have faded now that Macedonia is independent, but Sofia is still one of Yugoslavia's more prickly neighbours.