Lots of factors at play here.

- type of aircraft - some smaller jets have 2 seats on one side and 3 on the other or 1 on one side and 2 on the other - makes a difference when travelling with others

- if you like gazing out the window, depending on takeoff/approach patterns, you might miss something cool to see if you're on the left rather than the right and vice versa. I've seen some very cool views of the Manhattan skyline while landing at LGA because I sat on the right that I otherwise would not have seen.

- direction of travel - this is my biggest one for me - if I'm taking a morning flight leaving from say ATL and going to SFO, I'm taking the left side so the sun isn't blinding me the whole way!