When Aulë attempted to make his own creation, the Dwarves, Ilúvatar seemed to not like this and questioned Aulë for doing so.
Was it because Ilúvatar decreed Elves were supposed to be the first of his creations to awake in he lands of Middle Earth?
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Because Aulë tried to create rational life.
In Tolkien's (I will remind you, intensely Catholic) morality, attempting to usurp Ilúvatar's authority, or bypass his Plan, is a Bad Thing. The published Silmarillion isn't clear on this, but Tolkien writes it more clearly in the unsent Letter 212 (emphasis Tolkien's):
Aulë [...], one of the Great, in a sense 'fell'; for he so desired to see the Children, that he became impatient and tried to anticipate the will of the Creator. Being the greatest of all craftsmen he tried to make children according to his imperfect knowledge of their kind. When he had made thirteen, God spoke to him in anger, but not without pity
The One rebuked Aulë, saying that he had tried to usurp the Creator's power; but he could not give independent life to his makings. He had only one life, his own derived from the One, and could at most only distribute it. 'Behold' said the One: 'these creatures of thine have only thy will, and thy movement. Though you have devised a language for them, they can only report to thee thine own thought. This is a mockery of me.'
The Letters of J.R.R. Tolkien 212: To Rhona Beare (unsent). October 1958
We can contrast this with Yavanna, who, though desiring creatures of her own, asked for permission before creating them, and had a very different experience from the process:
So I see in my thought. Would that the trees might speak on behalf of all things that have roots, and punish those that wrong them!
Then Manwë sat silent, and the thought of Yavanna that she had put into his heart grew and unfolded; and it was beheld by Ilúvatar. Then it seemed to Manwë that the Song rose once more about him, and he heeded now many things therein that though he had heard them he had not heeded before. And at last the Vision was renewed, but it was not now remote, for he was himself within it, and yet he saw that all was upheld by the hand of Ilúvatar; and the hand entered in, and from it came forth many wonders that had until then been hidden from him in the hearts of the Ainur.
Then Manwë awoke, and he went down to Yavanna upon Ezellohar, and he sat beside her beneath the Two Trees. And Manwë said: 'O Kementári, Eru hath spoken, saying: "Do then any of the Valar suppose that I did not hear all the Song, even the least sound of the least voice? Behold! When the Children awake, then the thought of Yavanna will awake also, and it will summon spirits from afar, and they will go among the [animals] and the [plants], and some will dwell therein, and be held in reverence, and their just anger shall be feared.
The Silmarillion III Quenta Silmarillion Chapter 2: "Of Aulë and Yavanna"
Unlike Yavanna, Aulë acts almost Melkorian, independently trying to create something as great as the Children of Ilúvatar.