Why Did Uzzah Die For Touching the Ark?

[Metis]

I remember being told, in childhood, that whoever saw god died on the spot.

Then that means everyone that goes to heaven will die.  

I think he's talking about the concept of the "Beatific Vision". Most Christian denominations, Catholicism especially here with Purgatory, state that nothing unclean/with sin can come into the presence of God. To behold God, to be inducted into the Glory of the "Beatific Vision" you have to be judged to be righteous/sinless. If you're in heaven that means you're without sin, at least serious ones.

It's not that you'll die if you see God in his true form, it is more you simply won't be able or allowed to unless you fulfill the criteria.