very good question.
the answer is to that the potential across the sries/parallel resistors should equal that across the battery.
Why this is so is explained below.
suppose we have the simplest circuit. a battery of V volts and a resistor R connected to its terminals.
charge always flows from higher potential to lower potential.
why this happens is because;
suppose you are in empty space and there is just one charge in front of you, say a +ve charge. now if you keep a -ve charge in front of it, and suppose you clamp the +ve charge so that it can not move, the negative charge will move towards the +ve charge and will gain some velocity. this gain in velocity or kinetic energy comes from the loss in potential energy.
or suppose you take the negative charge and move it away to a large distance away from the +ve charge. there is a force on the -ve charge from the +ve charge and you have to work against that to move it away. so you actually worked and that work is stored as the potential energy of the negative charge.
as sson as you leave it, it will start moving towards the +ve charge and will gain K.E. and will lose all the potential energy.
this is what happens in circuits. batteries create an electric field in the wire and electrons rush towards the +ve plate and reduce their potential.
if the potential isn't same b/w any two points, the charge will flow from the higher potential to lower potential.