Covalent bonds can be strong or weak, that depends on the size of atoms in the bond and the number of bonds between the atoms.
Carbon atoms are small and then there are 4 valency bonds between 4 carbon atoms and the central carbon atom. This is because valency of carbon is 4. The electrons are shared 50% by each. So the covalency bonds are very strong. They are arranged in a tetrahedron structure. The carbon atoms are packed very closely and the gaps between the atoms is less because the atoms are small.
Thus the packing density of carbon atoms in a diamond results in the most stable (after graphite) and strongest substance.
Heavy atoms in covalent bonds are not so tightly bonded as the sizes of the atoms are large. So electrons are loosely bound to the nucleus. So the electron affinity will be less than that for a smaller atom. For florine, being very small, the electron affinity will be very high, for example.