Alternative views - Starts 2 first coz...
The bleed sides can then be split allowing pack on in hot weather whilst starting the other one.
The APU consumes fuel from the No1 tank, starting 2 first takes a little fuel from the no2 tank balancing things before you go.
Coz its always been done that way.
The first officers ears are less important than mine (who said that - speak up man, I'm deaf!)
On the 737 in all reality, it doesn't make a jot of difference. On the bus it does as the brake system gets topped up before the engine develops any meaningful thrust. I believe some of the bus series start the other one first because the system is the other way round.