One of the dark sides of Cloud's personality is his conviction of the fact, that he is SOLDIER 1st Class. At the beginning there is no any doubts that he is really a member of that elite Shin-Ra battle unit: he wears uniform of SOLDIER 1st Class, he has a sword that only SOLDIER can manage with, he has so-called "Mako eyes". Cloud's is the following: he was a SOLDIER 1st class, but now he is withdrawn from business and works as a mercenary, since his power and SOLDIER's skills allow to do much. Everything goes well, but gradually some facts are clarified that prejudice this story and make us think why Cloud has all attributes of a real SOLDIER.

We can begin from the fact, that Cloud tells to everyone around about how SOLDIER fights, however his own skills an the beginning of the game are quite low. As an example: Cloud's level at the moment of the begging of the Game is 6, and in his memories about fight with the dragon 5 years ago Sephiroth, the real SOLDIER 1st class has 50th level. Even if we take into account the fact, that Sephiroth is the best and the most powerful SOLDIER member, that means that the level of other 1st Classes must be at least, for instance, 25. How is it possible, that Cloud with such low level of training was able to be accepted to an elite unit, known to accept only best of best of best?
Also the dialogue between Aerith and Cloud about SOLDIER is notable. Aerith says about Zack as about SOLDIER 1st class, but Cloud answers: "Strange, there aren't that many who make First Class, but I've never heard of him". We know for sure that Zack was in SOLDIER, and Cloud, if he was there too, must know him. However, he doesn't.
Furthermore, Cloud himself does not remember how he joined SOLDIER. On the way to North Crater he remembers this:

"I spent the night and went to the reactor in Mt. Nibel. I was excited about it."

"Because that was my first mission after becoming First Class in SOLDIER."

"...SOLDIER, First Class?"

(shakes his head)

"...SOLDIER? When did I enter SOLDIER?"

(clutches his head, trembling)

"How did I join SOLDIER?"

(falls to the ground, shaking harder)

"Why... why can't I remember?"

The question appeared after comparison of these facts can be formulated like this: why Cloud is so powerful in theory and so week in practice? As if he was a witness and listener of how it is to be a SOLDIER 1st Class, and remembered everything so much good, that is able to earnestly repeat what he saw and what he heard. But outside of this Cloud can say nothing - and it scares him, as we can see from the quotation above. Other characters also points to this strangeness.
Image of SOLDIER 1st Class is supplemented by the uniform and by the sword. However they are not owned by Cloud. After escape from Hojo's laboratory Zack changed Cloud's clothes to this uniform, and the sword Cloud received after Zack's death. However, he doesn't talk about origin of these things himself, and people around him can not know that, and accept these attributes as a proof of the fact that he really is a SOLDIER member. Nevertheless, we see, that presence of these things in Cloud's hands is evidence against of the fact that he is a real SOLDIER 1st class.
Third fact that is an evidence of Cloud's membership in SOLDIER is Mako eyes. If Cloud didn't take a treatment of becoming SOLDIER - how his became like this? We remember that after tragic events in Nibelheim Cloud was a subject of experiments in Hojo's laboratory targeted to create Sephiroth's copy. These experiments in the point of fact were the same as a process of giving SOLDIER's abilities to a man. Jenova cells and Mako separately calls mutations and Mako poisoning, that is why Hojo in his experiments used combinations of these components. In that way Cloud obtained Mako eyes - not in a procedure of becoming SOLDIER but in the procedure of becoming Sephiroth's copy.

Collecting all worded facts together we get that Cloud is well-informed about SOLDIER and Mako eyes is a result of experiment that has no relation with joining SOLDIER. In that way we see, that Cloud is not a SOLDIER member. In AC Rufus calls Cloud as "would-be SOLDIER" and Cloud does not disputes about it. Lets try to investigate how does it happen and why Cloud pretends to be someone that he is not, and who is that "someone".
We remember that after Nibelheim incident, finding himself in Hojo's laboratory Cloud appears to be a success result of the experiment, a dummy ready to accept any information. According Hojo's plan, such dummy, or marionette, must become a Sephiroth's copy. However, after escape from the laboratory he beard strongest shock: Zack's death before Cloud's eyes. This defined which information was accepted by destroyed Cloud's personality.
Cloud does not begin to consider himself as Zack. He as if replaces the one who dead with himself, becomes a copy of Zack instead of Sephiroth's copy. There was no Zack, SOLDIER 1st Class, Sephiroth's friend, there was Cloud, SOLDIER 1st Class, Sephiroth's friend. He accepts as an instruction of further life the story about "former SOLDIER, currently, a mercenary" heard from Zack, he has the same manners, he has fun in the same way - by doing squats, he is always comes late as well.
In AC we can see the moment: Cloud stands on Zack's grave, than poses as he poses in the Game, and pronounces the phrase: "I'd live out both our lives ...that's easier to say than do.". After that he shakes his head as if tries to banish from his mind a delusion. Why? "It's easy to talk than to do" is a phrase of Zack from FFVII, and the pose Cloud usually uses is habitual Zack's pose. Cloud in AC, understood what has happened with him, trying to be himself, searching for his personality, can not get rid of image of SOLDIER 1st Class Zack, imposed in him, and it inflicts true suffering on him. In the Game also there are moments where Cloud repeats Zack's words and behavior, however at that time he didn't realize that these words and manners didn't belong to him.
It is no wonder that people who met Zack feel déjà vu , and the most strongly similarity of Cloud and Zack strikes, of course, Aerith.
To speak about Aerith being in love with Cloud - that means to make a serious mistake and to distort a beautiful and tragic story line of love between Aerith and Zack. Relationships between Aerith and Cloud build up exactly on Cloud copying Zack. Aerith never loved Cloud, she was charmed by image he created, image that remembered her about her beloved. Aerith strictly says this during their date in Gold Saucer:

.........first off, it bothered me how you looked exactly alike.

Two completely different people, but look exactly the same.

The way you walk, gesture...

I think I must have seen him again, in you...

But you're different.

As once Tifa understood that there is something critically wrong with Cloud, Aerith understands it too. In the same way as Tifa decided to look after Cloud, Aerith didn't let him go, literally thrusting herself on Cloud did not leave her, running from home during the night - everything only to not part with a man who looks so much like Zack, and who, perhaps, will tell something about him. This is the explanation of so-called "love triangle". This is not the matter of love, but the matter of fact, that Cloud causes painful anxiety to both girls, and one of them takes him for her childhood friend, and the other one sees in him features of her beloved man.
Conversation in Gold Saucer is the last one before events in Temple of the Ancients, and after that Aerith leaves party - it's a denouement of her relationships with Cloud, dotting "i's" and crossing "t's". Cloud is not Zack, and with all his resemblance, will never become him.

After conversation with Tifa in Lifestream Cloud himself confesses that his story about him being SOLDIER 1st Class and about course of events in Nibelheim five years ago is all lie. SOLDIER 1st Class Cloud does not exist and never existed. At the same time he for the first time says words "my friend, Zack" :

I never was in SOLDIER.

I made up the stories about what happened to me five years ago, about being in SOLDIER.

(.....)

...then I heard this story from my friend Zack...

And I created an illusion of myself and made up of what I had seen in my life.....

And I continued to play the charade as if it were true...