Why is it okay that I can have an atom bomb, but you can't?

Complicated matters, like Israel – Palestine for example, are…, well, complicated for me and usually I approach them ‘childish’ (like: how about Israel and the Palestinians each take a part of Jerusalem and share the rest of the area evenly and live together in peace from thereon).
The same childlike view I have on the nuclaer bombs controversy.
Can someone explain to me why it apparently is okay for, lets say, France to have an atom bomb, but not okay for, lets say, Nigeria to have/built one?
Or America yes a bomb, and Iran no a bomb?
Who are France and America (and a great number of other countries that support that opinion) to deny Nigeria and Iran (and probably some other countries that develop or have these bombs) from making or having nuclaer weapons?
I assume geo-politics and/or economy and/or safety and/or religion have to do with this, but I am looking for the basic explanation.
Little Jake made a knife from a branch. Now Peter wants to make one too. Jake says no way to Peter. Do it, and I will stab you.
That sounds so wrong, or am I missing something?