Methodist Scott McKay, writer of the "Grace Happens" blog, says that unity is not the same as no disagreements. And disunity sneaks in with pride, jealousy, resentment, ego, and a desire for control.
Though Scott, a member of the clergy, associates such problems with the church membership, these same unity challenges pertain to all people associated with a congregation, clergy included. None of us are immune to the sins of pride or arrogance (for example), though some people do better than others in staving off these diseases of the soul.
His posting ends with a reminder of the invitation to Communion ... love, repentance, and peace.