Why re-boiling milk doesn't boil over again?
If you heat food high enough, you can cause chemical changes.  Letting it cool back off doesn't necessarily reverse these changes.
Typical changes include:

deactivation of enzymes
denaturing (breaking down) proteins
converting sugars into more complex forms
evaporation of part of the liquid

In general, we call this 'cooking', but there's specifically a term in English for that you're doing to the milk: scalding
I suspect that you've denatured the proteins in the milk, and proteins can create foams (such as what happens when you whip egg whites or the liquid from canned beans), but there might be other processes that have happened as well.
I also know that scalding isn't called for typically in modern cookbooks.  Part of this is because today's milk is homogenized and pasteurized, and so some of the chemical changes that we get from scalding have already happened.  But there are still quite a few people who insist that you need to scald milk for certain types of bread making, or you won't get the proper rise and final texture from the loaves.