$S$: a smooth projective surface over $\mathbb{C}$ which has non-negative Kodaira dimension.

$L$: an ample divisor on $S$

Why $K_S.L\ge0?$

I know that :

for some $m\ge1$, there is an effective divisor $E \in |mK_S|$ s.t.$0< E.L=(mK_S).L=m(K_S.L).$

Why is "$=$" possible?