They were allies because both Hitler and the Bulgarians had something to gain out of an alliance. Hitler needed Bulgaria as a base to secure the Balkans, and as a launching point in case of an attack on Greece, which he emphasised in his Directive No. 18. Bulgaria on the other hand saw the alliance as an opportunity to reclaim territory lost during the First World War, namely parts of Thrace, Macedonia and Dobruja.

As Bulgaria was a slavic nation, Hitler would have had a low regard for the Bulgarians, however this was superseded by the usefulness of Bulgaria's geographical position. The relationship between the countries was not close, and King Boris refused to deport Bulgarian jews after public outcry.

Sources are Ian Kershaw's Hitler and John H. Waller's The Unseen War in Europe: Espionage and Conspiracy in the Second World War

According to Speer in private conversation Hitler said:

"It's nonsense to think that Bulgarians are Slavic".

Hitler also said in speech that "The west has never been fair to Austria, Hungary and Bulgaria".

In another speech Hitler said: "Bulgaria has always been loyal to the German reich".

Finally in 'Mein Kampf' Bulgaria gets mentioned only once in neutral context. There is no reason to believe that Hitler regarded bulgarians as "slavs"/enemies.

Bulgaria also was against the invasion of the soviet union and against the deportation of the jews which the Nazis somehow accepted or at least tolerated to an extent.