During my student years I mostly flew whatever the cheapest airline was, with a strong preference for United because it helped me get status on bmi for relatively little travel.
Then at one point I flew BA to the states and my seat was chosen to do some survey and I signed up for BAEC at the same time as it wasn't a hassle. I was eventually able to roll my remaining bmi miles into that account, so it felt like a natural progression.
I have never flown enough (or in high enough fare buckets) to make it worthwhile to be in two frequent flier programmes, so at this point, I fly BA whenever possible, but otherwise it's the cheapest airline without any particular preference. (North American travel is nearly always BA, but eastwards flights have been a mishmash.)