Why do people put up Christmas trees?

Prince Albert (husband of Queen Victoria) is often associated with the rise in popularity of Christmas trees: https://www.townandcountrymag.com/society/traditio...

Whatever the Queen and Prince did -- often sparked a rise in interest in whatever they were doing, wearing, eating, etc.  The Christmas tree is an excellent example of that. 

Because they are pretty

Because you can't pu them down.

There is a passage in the bible that talks about this, it is considered a pagan idol and the Jewish God condemns this practice, but in Christianity he was accepted among other pagan symbols, not for heresy, but for the conversion of the people.

Its linked to an old pagan tradition relating to the winter solstice, bringing a living tree into the house and paying it reverence to ensure the winter passes. Putting the topper on last is just human behaviour, finishing with the most importany part of the task because it feels right. It would be weird to finish with some random baubel a 3rd of the way down the tree, although i doubt anyone could fully articulate why

My guess is, that its because the tree is an evergreen, representing life