Said Rabbi Yosef Yitzchak of Lubavitch:
Their are many doors, but doors have locks. One needs keys, many keys, to unlock the doors.
But there is also a master key, a key that fits all locks and unlocks all doors. The master key is the chassidishe niggun, the chassidic melody.
Adapted from the teachings of the Lubavitcher Rebbe by Yanki Tauber