Why is the current I equal and the voltage u always = U1 + U2 +... UN when the circuit is in series

Why is the current I equal and the voltage u always = U1 + U2 +... UN when the circuit is in series

U total = IR total = IX (R1 + R2 +... RN)
U total = IR total = IX (R1 + R2 +... RN) = IR1 + IR2 +... + IRN = U1 + U2 +... UN