I got a good deal of passion to serve Hashem once I was convinced that He isn't disgusted with me at all. Think about it... If a rebbi, teacher, parent, sister, wife, store clerk, whoever... have this look of near-puking whenever you see them looking at you, would you be able to summon up the resolve to give them your all? Or to deal with them with 'passion'? I doubt it.
If deep inside I really believe that He "knows" that I am a dang loser, the passion ain't happening. (Same goes for my wife in the relationship... k'mayim ponim el panim., etc...) Once I came to believe that Hashem truly loved me with a passion, even in the midst of my insanity, I was able to begin to serve Hashem with love and passion as well.