I fly NW because of it's hubs. I don't have to connect through IAD, ORD, DFW, LAX, SFO, or JFK. DTW and MSP are great places to connect through, maybe except in the winter

I fly NW because I like quick connection times. I don't want to be hanging around in an airport for 5-7 hrs.

I fly NW because they have lounges that don't offer drink passes/coupons. I want to drink all I want and not be limitied.

I fly NW because I do business in Asia and their flights to Asia are fantastic (outside of no NRT-TPE).

I fly NW because it just works for me.