Training in the martial arts, we begin and end with respect!
Uke and Nage share a training partnership--not a competition.
The uke attacks and the nage (or tori) defends. The senior student in a pair is nage first, unless directed otherwise. Typically, the uke will attack four times, changing side each time. The uke should attack in response to the stance (kamae), or target offered, of the nage. Nage should, for example, set up to invite and attack to the left side and then change sides for each subsequent attack. After four executions, uke and nage reverse roles, unless sensei has given a student instruction, in which case it is appropriate to take a couple more executions before trading roles. Force (strength and speed) of the attack should be appropriate to the nage's skill level and the stage of learning at which the nage is engaged.