DL does not have my "loyalty"...as I only fly them when it makes sense (For example, I am flying Spirit for the first time next week to save $500
)
However, my wife and I highly enjoy the high number of x2 seating arrangements on aircraft (MD-80,90,717,767,330,CRJ700/900, etc). Sitting in Y with just your wife makes a flight endlessly more pleasant than with a stranger.
We also fly to the Gulf Coast a lot and pricing, equipment, and frequency with DL cannot be beat.
That being said....Skymiles is now absolutely worthless for leisure customers.