Question about proton and neutron interaction.?

First of all I only want people who really now what they are talking about answering this question. I was wondering, why is it so uncommon to have stable isotopes with an odd # of protons and an odd # of neutron? What physical property makes this happen? Does it have to do with atomic forces like the strong force? I have been wondering this for a while but have been unable to find the answer. Specifically Tc #43 on the periodic table.