Looking at a graphical representation of electronegativity in the periodic table reveals a pattern that, noble gases aside, electrognetivity increases as you move toward the upper right hand corner of the table. What causes this? The electronegativity seems to anti-correlate with the empirical atomic radii, which doesn't correspond perfectly with the calculated radius, so I would imagine that the explanation of these phenomena are shared, for what it's worth.
Mendeleev's periodic table of elements is based on the number of protons in an atom. Also, in an electrically neutral atom,
$$no. \ of\ electrons=no. \ of \ protons$$
So for stability, octet configuration is sought by the atom. For electro-negative elements, since they have a higher number of electrons in the ultimate shell, they tend to gain an electron to form the octet. And since according to Mendeleev's arrangement, atoms with more number of electrons in the ultimate shell are placed on the R.H.S.
But, the most electronegative are at the upper-right, because they tend to be smaller atoms by size and the nucleus exerts a greater pull on the outer electrons and hence can attract electrons to form octet configuration more easily in comparison to larger radii atoms, making them more electronegative.