We both set the tone. We're both a little moody and prone to getting down and/or grumpy sometimes. We also pick up on one another's state of mind, and it all goes downhill from there. As soon as one of us starts to go downhill, it falls apart a bit for a while.
In my childhood and my first marriage, the man tended to set the tone - but that was because my dad's an alcoholic, and my ex was an emotionally abusive drug addict. With people like that around, everyone else tends to wind up reacting to them all the time.