Live (balled) Christmas trees are very heavy (because of the root ball), and the root ball may contain mold spores, and if one is allergic to mold, one doesn't breathe and one has a headache throughout the entire holiday and might be snarly and not in the Christmas spirit at all because having a headache and not being able to breathe is not fun.

And then when the tree, which can stay indoors for only a week without damaging the tree, is planted outside and dies, one who couldn't breathe and had an aching head all through the holidays might wonder why she even bothered.

Not that I know anyone whose ever had such experiences or anything....