If you read Kant thoroughly, and agree with his starting points, he explains that 'necessary' becomes logically obvious.

We humans are cursed/blessed (depending on your attitude) with the *ahem* ability to ignore logic and do as we see our own advantage.

This is SUCH a hard point to make to those whose own argument forms are already outside of logical forms. They write their own rule book, then follow it....ummmm....religiously.