Good question, pgupta011! Part of the reason I've been flying UA lately is the hub factor. As you know, IAD is a UA hub and thus offers numerous flights to just about anywhere. DC is not very well served by CO. In fact, the majority of my CO flights have been shorthaul from DCA to EWR (CO is the only one who serves that route). Schedule is also a big factor. I was recently commuting on an almost weekly basis to YVR. Back in February when I realized I was going to be headed to YVR regularly, I looked at schedules on all the major airlines and found UA's to be best (CO and AA ran second). They provided the opportunity for me to arrive at a reasonable hour in YVR on Sunday evenings and return into DCA (via ORD) before midnight on Fridays. Both AA and CO would have required significantly more travel time at worse hours. Also, I believe the IAD double bonus miles offer had been announced and I realized this would provide a great chance to rack up miles! It worked out rather well. I will not come anywhere close to elite on CO this year (I think have 8 segments this year), although I will hit 1K on UA and will retain Preferred on US (30 segments).