A "groove" is what you had on those old style records. The needle moved in a groove. "To be in a groove" is taken from there. It may be the groove of spiritual balance, or it may be the groove of a dull routine. Generally speaking: 1. In THE groove = something nice 2. In A groove = something boring

Both of those definitions are related, even though they may sound like different definitions. Picture this: You're listening to some good music, and you begin to snap your fingers according to a certain rhythm. That's what you call groove...it's a type of enjoyable routine. So, groove is used when you are describing a routine or rhythm that is positive (as opposed to negative). For example, "It will take me a few days to get back in the groove". People do sometimes use it in a negative way, like, "If we don't help him, he will go back into the same groove". However, in everyday conversation the word is almost always used positively, at least in my experience.