The cause of why the North Atlantic (NA) blocks are more frequent and long-lived during the negative phase of the North Atlantic Oscillation (NAO) than the positive phase is theoretically explored using the envelope soliton block-eddy interaction model. It is shown that the negative NAO phase can enhance the planetary-scale split-jet (diffluent) flow prior to block onset, but the positive phase tends to weaken such a flow. Through the interaction with upstream synoptic-scale waves, the enhanced diffluence in the background flow during the negative phase of NAO allows the isolated block to form more easily and live longer. Thus, it appears that the negative NAO phase can provide a favourable environment for the frequent occurrence of the NA block and its persistence.