The main reason for this phenomenon is due to the electronegativity of the atoms involved. The trend for electronegativity is up and to the right on the periodic table with Fluorine being the most electronegative. The two main contributors to electronegativity are atomic radius and the number of protons in the nucleus. Moving up the periodic table leads to a smaller atomic radius and stronger electronegativity while moving to the right increases the number of protons. More protons means a stronger pull on electrons within the compound, and a smaller atomic radius means that the protons are more able to interact with the valence electrons. These two aspects lead to valence electrons being pulled more towards one atom over another.