Well, it's because of two things. First, the number of protons or number of protons found in our nucleus increases as we go from lithium to fluorine. And second, the number of electrons found on our most electron shell also increases. And this means, according to Coulomb's law, the force also increases. In other words, the force with which the protons pull the outermost electrons increases. And this means that our effective nuclear charge on our atom increases.