State of disorder is much more likely than the state of order, use for example the experiment with the numbered balls in a box, when you shake the box the probability that the balls after that are in the same sequence like at the beginning is very low. It is the same with chemical equations, the reaction will happen easier when a bigger disorder is going to be made (dS>0). If you know this, and that the lower the Gibbs energy is (dG<0) the equation is more likely going to happen then. I think that's why the ''-'' is there. Bigger disorder, entrophy more positive, Gibbs energy lower and the reaction will happen easier.