Why it's normal to be nervous in public speaking situations?

Nervous public speakers exhibit any number of ticks and tells. While teaching public speaking, I isolated a number of common mistakes public speakers make, each or any of which can be strong sign of nervousness as well being unprepared. The examples below in quotations are names I have given the traits:

1. "Dancing Feet" - Shuffling feet, often heel to toe.
2. "The Great Flood" - Excessive sweating
3. "Water!" - When water is made available on the lectern, some will go after it like a man coming out of a desert after a week without.
4. "Bird Hands" - This is a matter of the speaker's hands fluttering around like a startled pigeon.
5. "The Jitters" - Marked by excessively shaking hands, or shoulders. The speaker will practically vibrate.
6. Stuttering - A clear sign the speaker did not prepare
7. "Um-Uming" - Um, um, um, uh, uh, um, well, um, um.
8. "Read Me A Story" - The speaker reads directly from index cards, or a pages or pages.
9. "Death Grip" - Clenching the podium or lectern
10. "Flamingo Feet" - Standing on one foot with the other foot nested behind the knee of the supporting leg
11. "The Perfect Storm" - Rocking or swaying while maintaining a death grip on the podium or lectern.
12. "Puppet Hands" - Mindless hand gestures, almost appearing to be suspended from strings.
13. "Whiplash" - Favoring one side of the audience, head turned and locked.
14. "I Can't See You" - looking at the audience, but never connecting.
15. "Locked Gaze" - Focusing excessively on one audience member - Staring
16. "What's Up There?" - Focusing on the ceiling (Like no eye contact but more of a Locked Gaze)
17. "What's Down There?" - Focusing on the floor (like no eye contact but more of a Locked Gaze).
18. "Averting eyes" - similar to no eye contact but constantly moving the gaze all over the room
19. "Nervous Feet" - Foot bouncing, tapping, jiggling. This is similar to Dancing Feet, but the toes never break contact with the floor.
20. "The Big Shrug" - hands deep in pockets with shoulders in up and tight
21. "the Thinker" - an odd placement of hand on face to mimic Auguste Rodin's famous sculpture.
22. "The Long Walk" or "The Caged Tiger"- stalking back and forth in front of your audience, sometimes pounding out the points of the presentation with your steps
23. "The Moving Target" - similar to the Long Walk, but with much less predictable pacing, more erratic movement. In a classroom, or conference room, the speaker may move around the entire room seemingly with no destination in mind.
24. "Machinegun Mouth" - The speaker delivers the speech in stacato burst, rarely changing tone. It will have the cadence of a gunner firing three to five round bursts.
25. "I'm Shy" - Hands in the pockets, head on the shoulder, eyes cast downward, and there may even be a glow of blushing. Often the shy speaker will shrug in this position, and may even sway side to side, rotating back and forth.
26. "Turtle Head" - More common with male speakers, the speaker will stretch and twist the head and neck similar to that of a turtle coming out of his shell. Could be the symptom of a too tight collar too.
27. "The Samuel Johnson" or "The Daniel Webster" - You speak like you ate a dictionary, and use Words that you are not even certain of the meaning.

It is important to know your material. It is important to practice your delivery in front of a mirror or a video camera, best yet with one other person there who can provide feedback. Most importantly, the speaker needs to know that while he is in front of his audience, he owns them; their time and attention are his to do with as he pleases (to some extent). The speaker is in charge. Society is such that politeness is driven into each and every one of us, and it is hugely rude and insulting to intrude on the speech of another. There is nothing to fear. Likely there will be no cabbages or rotten tomatoes that will be flung from the audience at the speaker. He will not be dragged from the stage and burned at the stake--probably. Know your material, and be in charge. Talk to your audience as you would a group of close friends. Just keep the slang and group speak to a minimum--like zero. Use proper speech, and don't try to talk over your ability; use Words you would use. Impress your audience with what you know, deliver it in a relaxed way, and don't try to impress them with new Words you just looked up.