Commercial Fixed Wing
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Distraction; Situational Awareness; Time Pressure
Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew Rejected Takeoff
Environment - Non Weather Related; Human Factors
We taxied out single engine.  There was some distraction/discussion on taxi out regarding a large brush fire burning to the east of the airport.  Upon turning north to Runway XXL we noticed Aircraft Y holding short of the runway. There was some discussion of the aircraft as we had a similar setup the day prior in ZZZ and it changed up the departure sequence and we had to maneuver around that aircraft.  As we neared the runway the Tower then cleared as for departure without delay as there was another aircraft on final.  We lined up on the runway and as I added power I noted the asymmetrical thrust. I rejected the takeoff and advised Tower. After clearing the runway we started the second engine and completed the takeoff without incident.In my opinion we developed some expectation coupled with distraction that it may be a bit before we departed. This must have pulled my mind from the task at hand (starting the second engine).  With the Tower then expediting our departed we switched into a rushed sequence that resulted in the error of not starting the second engine.  The other aircraft and the aircraft on final also pulled my eyes and attention outside the aircraft more than usual.In hindsight and trying to examine why this happened I have also had some personal distractions that could have been a casual factor in contributing to my lack of focus.  This was my first trip after recovery from a moderate case of COVID as well as an [another personal issue.]  While I can say these weren't outwardly conveyed or discussed they were heavy on my mind.
We taxied out to XXL from the gate single engine. On the way we noticed a big smoke plume and also saw a fire truck spraying water on field which was unrelated. When we turned on bravo away from view of the smoke we noticed Aircraft Y awaiting takeoff similar to yesterday at what appeared to be the end of the runway. We couldn't tell they were at the intersection prior until we were upon them.  I was looking at them when Tower cleared us onto the runway. I read the line up and wait back and instinctively started my before takeoff flows. Notifying the flight attendants; transponder and to config. As I read that checklist ATC interrupted and said cleared for takeoff. I read that back then restarted the checklist and finished it. The captain added power and immediately rejected. We notified ATC and exited the runway. We then started the engine and ran the checklists from there and resumed the flight.
Air carrier pilot reported asymmetrical thrust on start of takeoff roll resulting in a rejected takeoff.