what is the reason behind the Faceless soul's mood swings?

No Face believed that he had to buy friendship and respect. And he does, and grows from it, he feeds - sometimes literally - on other peoples attention.
The first turning point comes when Chihiro refuses to accept his gold offers, and instead offers him the bitter medicine. No Face is infuriated when the bitter pill causes him losses when he vomits. The second turning point comes right after, when they enter the train, and No Face is offered a ride whitout having asked for it, and without paying for it.
Now, he is cured, and find peace.

For the heart life is simple, it beats as long as it can.

He also fed on the greed of the staff, pumped them full of the sin of greed (fattened them up so to speak) then consumed them

I think this is better understood from a psychological perspective in that, as is said in the film, when he is inside the place he turns into a monster, so in my opinion this is more artistic expression of saying that the lower aspect that seem veil at first may actually only be that way because it's being ignored and treated like a monster. The parts of you that you reject in the world and in your personality hold a treasure inside and need to be set free from biases and one sidedness towards them. It is calling your attention and wants you badly to notice it. It reminds me a lot of the Satan concept where since many Christians put all the evil outside of them in an evil being they shove out something of themselves and fail to see that maybe whatever it is they think evil may actually hold value or in the case of their sexual suppression, turn into something like sadism through unconscious distortion, in the case of sadism or any dysfunctional personality you can label your faults as bad but look to see what is really at the center trying to get your attention like this dark character tried to do so badly. You can't ignore what appears at first to be chaotic and not jive with your conception of yourself and the world because it just splits off a part of your personality whether it's inherently good or bad from a moral perspective and even if its bad maybe its only that way because you don't look at it and see what it is trying to tell you.

You'll also realize that it was when she didn't pay attention to it that it became a monster. The point is just hey im a part of you and you rejected me and im going try extremely hard to get you to look at me whether it be gold you want, or whatever. She is the only one who sees him for what he is and its in a way the most important part of the story as many storied have this myth inside a myth format where the lesser one is rarely understood by an audience. This story reminds me of the odyssey.

Im sorry cause I hate when people respond so many times but we see at the beginning that she is a very faulted character who isn't even likeable much and that's when we should know that the story is going to be about he developing soulfulness and character through a process of growing into her personality. Which means there is possible going to be a confrontation with a dark figure at some point and hence we have it.

Well, I think I can help you with a little bit culture background.
A face and a name is very import in Asia especially Japanese culture. A real name is one of the strongest curse and protection curse. That is the reason why Yubaba changed Chihiro (thousand research) into Sen(thousand) and stole Haku's name, so she can control them.

Also, a solid face represents a solid spirit. That's why Haku told Yubaba she cant see what is most important to her, she got so greedy into money that she could not see through a simple magic her sister cast on the her baby.

Now comes to your question. The no-face does not have a face(that's why he wears mask) nor a name. That's why he were never able to get into the bath house( only stay on bridge and in the garden), since Yubaba considered him as evil and dangerous because she cannot see through him or curse him. Reflected to reality, he is not somebody, he represents loneliness, numinous loneliness.
Chihiro did not know that he is forbidden to get into the bath house, and she greeted him and let him in. She was the first person who cared about him and opened her heart to him (even though by accident), she gave him attention. Then later he found out that giving Chihiro token made her happy and grateful, so he stole more to tried to make her happier. Even though she rejected, he still gave them to her, to wish her happy and wish her happiness can made his loneliness go away. But later he got the false impression that money will make people happy and people with money will be respected. (He watched people's attitude changed throughout the whole event of cleaning up the river spirit.) So he tried to use money to get respect and attention . However, money cannot made his loneliness truly go away. That's why he kept getting bigger and bigger since he became greedier and more violent and yet still lonely.(that's why later all the money he gave out turned into dirt) So he wanted Chihiro, he thought Chihiro can make him lonely no more. When Chihiro rejected and run to help Haku, he got extremely aggravated. Whatever Chihiro fed him, had a strong spell that made people turn back who they were. So he started to vomit out all the bad things he swollen down to kill his loneliness. That's a extraordinarily painful process that he wanted to kill Chihiro. But after he got off the bath house and spilled out the last negative stuff he took, he turned into who he was.
He got on the train that never returns(resembled life, since life is a single direction journey) with Chihiro, because that was one of the few things we have throughout our lives: speechless loneliness.

He is shown to be capable of reacting to emotions and ingesting other individuals to gain their personality and physical traits: http://spiritedaway.wikia.com/wiki/No-Face_%28Kaonashi%29