Why is an interviewee advised to wear a smile in an interview?

Anny: Hi there. I’m Edith Richardson and this is Howdini. com. If you wanna learn how to ace a job interview, stay tuned. We’re about to talk about it with Maggie Mistal, who was a life purpose and career coach, who can be heard regularly on Martha Stewart Living Radio on series. Thank you so much for being with us.
Maggie: Thanks, Edy.
Anny: You show up your dress properly, you walk in to inner to meet the interviewer... What’s the first thing you do when you walk in?
Maggie: I love to smile, and obviously I think you can tell by that. But I like to make the interviewer just as Edy is, I’m trying to be, because really this interview is a conversation that you’re gonna have with the other person, who is a human being. And it can be a real icebreaker if you can just be relaxed at least gives yourself a break. Say "hi" , "how are you?" and even try to talk about the weather or some type of other ice-breakers that can give everybody a chance to take a deep breath. Because this is invaluable to take a conversation and it can be nerve-wracking for both parties.
Anny: So the interviewer says to that perspective employee "Why do you wanna leave your job?"
Maggie: Yes, that’s always a great question, isn’t it? And you have to be prepared for those key questions, especially why you are moving on. Now if you’re moving on because you can’t stand your boss,(right?)which happens a lot and we’ve all been there. You don’t want to say that. What you do want to say is that " I really need to be in a place where I can be managed and developed and where I can get guidance or mentoring". You really focus on anything that you’re looking to get as opposed to all the reasons why you don’t have it now.
Anny: There are some people who walk into that interview and they walk in a little defensively, and they... because they really need a job, and they hear the words "You’re overqualified" and they’re willing to take a job and do their best at it. How do you handle " You’re overqualified"?
Maggie: You can say things like... well, I’m... you know I really thought about that if I was in your shoes I should be asking the same question. And when I think about it I can tell you for a fact and I’m looking to downshift. And I know this industry well and I don’t need to be in charge. I’ve been the person on the totem pole before. I can do that again. I’m really looking for a place where I can contribute but I don’t have to be the one who’s running the show. And(you)really address those concerns which are behind the overqualified combat.
Anny: And they may say "You just don’t have enough experience for us. "
Maggie: For folks just starting out may be a new career, or college students who just graduated or even folks at high school who needed the work world. That is a common problem. So a-gain I go back to... Internships are great anyway that you can get some experience on your resume to demonstrate "Hey yeah, I may not be, you know, 10 years in this field but I can tell you this experience that I had this summer. I didn’t know anything going in and here’s how successful I was. " You wanna show situations where... yeah... you went in without a lot of experience but were still able to succeed and this is for references can be so helpful as well. Even you don’t have a lot of experience: anybody can be great reference for you. Even a teacher, or professor or even someone that you volunteer for, who can attest to your ability to learn quickly and perform well can be a great asset.
Anny: "We really like you, how much money are you looking for?" And such a trick question.
Maggie: It is, and for that question you definitely want to do your homework. This is what the best uses for the internet: salary. com, indeed. com, monster. com, hotjobs. com... A lot of them have salary calculator associated with them now. So you can look up the job title, energy graphic area and get the range of salaries for that position and find where you would fit based on your level of experience.
Anny: Briefly if I... if the interviewer has brought up the word "salary" , should you?
Maggie: Not right away. In the initial conversation, you want to get to know whether or not this job even is a good fit. And I always tell people, become the superstar to this person first and then you’re in a much better place to negotiate salary and they’ll even pay more for you because I like... well, we can let the negotiation go such fabulous, can we? We are going to pay this much but now we find needs for willing to do more.