Going in the direction of loop if we move from +ve to -ve of the voltage(terminals of the voltage source) we take the negative sign. And we take it as a positive when we move from -ve to +ve terminal. 
Going in the direction of loop if we move from +ve to -ve of the voltage(terminals of the voltage source) we take the negative sign. And we take it as a positive when we move from -ve to +ve terminal.