The Bulgarian king during the onset of World War I, the German prince Ferdinand I (nicknamed "Foxy Ferdinand" for his ambition and slipperiness) was obsessed with gaining control of Macedonia, directly west of Bulgaria. This land at that time was controlled by the Kingdom of Serbia, an Entente power. Thus, after some deliberation, the king and government decided to go with the Central Powers. They enjoyed some success, but eventually ran out of supplies and were the first Central Power to surrender.

As to the second part of your question, the Bulgarian government at least was quite happy to ally (at least temporarily) with the Ottomans against the Entente powers, who were largely anti-Bulgarian. In fact, before the war even broke out, the Bulgarians and Turks signed a secretmutual defensive pact. I cannot speak for the average Bulgarian's opinion of the alliance.

In short, the Bulgarian government was willing to put aside their potential dislike for the Turks in order to acquire Macedonia from the Entente Powers.

My main source was "Paris 1919," Margaret Macmillan