Once upon a time, very long ago, Santa was having a very bad Christmas.
The reindeers had gotten into the eggnog and got themselves quite drunk. Rudolph's nose was bright red, and the reindeer were tangled in their traces. Then as Santa was loading presents on to the sleigh, the runner broke off one side, and dumped all the presents into the snow. So, Santa called for the sleigh jack, and jacked it up. Nailed the runner back on. About that time the other runner fell off, and dumped the presents into the snow on the other side.
About this time, the Christmas Angel came up to Santa and reminded him that there was no Christmas tree at the North Pole for Mrs. Claus. Santa had all he could handle, so he told the Christmas Angel to just go out and find a good tree, and everything would be OK.
There sat poor Santa with his head in his hands. No Christmas tree... Not enough gifts for boys and girls... The reindeer in no condition to pull the sleigh... and no sleigh to pull. Everything had gone very wrong.
Suddenly, the Christmas angel returned dragging behind him a HUGE Christmas tree."Hey Santa! I have your Christmas tree. Where do you want me to stick it?"
...and so began the tradition of the angel atop the Christmas tree.