Nostalgia
A family member of mine worked for CO for ~15 years and was there when Bethune turned it around. I fly CO out of Nostalgia, remembering how bad CO was pre Bethune and how good an airline it has been since. I fly UA as part of that legacy with the hope that the level of UA service improves to my perception of CO service. Generally speaking, I have found FAs on CO to be much more attentive and service oriented than UA. That said, both UA and CO have a long way to go to reach the level of service afforded on Emirates or Qatar airways.