Fearing Hashem has traditionally been spoken of as a central tenet in pious Judaism. One who fears Hashem is showing respect for Hashem and is more likely to live a pious life and not step out of bounds.
Now obviously, the translation for "fear" is not exact and the word likely reflects more of a fearful "awe" rather than pure fear. (like an existential fear with regards to existence or something immense) "Fear" is just the word which stuck.
The point I am getting at is how does the central Breslov philosophy of worship through joy and love coexist with a philosophy of fear?