I'm in a similar situation as the OP, though I have a 20+ year record of steady, heavy traffic (for domestic) with DL. I would have thought with today's sophisticated data mining I would at least have received a "we miss you" email. Nope. But I don't take it personal. Call me a sap but I feel loyal to DL just because I've lived intimately with them for 20+ years.

That said, I'm having a summer "affair" with AA having received Plat status from my corp travel department. My travel policy will pay for AA upgrade coupons so I've been up front on most flights. I must say AA does several things better than DL, but the main reason for this temporary switch is that I spend 4 months working out of CO in the summer and DL coverage there is weak. AA isn't real strong either but they seem to have some strategic flights out of COS and ASE that work for me. Last year I flirted with UA but that affair ended poorly.

While I DO know all these airlines are running a business, and that trumps everything, DL would enhance my loyalty even more if somebody noticed - but they haven't.