Why does Allah say "We" not "I" in the Quran?

It is really very strange to ask such a question and to think about why Allah says "We" not "I" though the Quran teaches oneness of Allah throughout.

It is really something that cannot be understood and explained for a Muslim to have doubts about the Quran by some questions whose answers are very easy though he needs to strengthen his belief in the Quran and his creed with each evidence.

This simple question may have harmed the minds and hearts of many people and caused them to have suspicions about the Quran. We prepared this work to help and cure those whose minds and hearts have been harmed by doubts and delusions.

Now, let us answer the question why Allah says "We" not "I". Let us find out about the answer in items:

1- The word "we" is a word used by kings and sultans. When they start speaking or writing letters, they start as "We, who…"
For, this expression includes majesty and greatness. Allah is the sultan of pre-eternity and post-eternity. To prefer the word "we", which expresses majesty and greatness, to the word "I" is definitely a necessity of majesty. 

2- Allah uses the word "We" when He mentions things in which angels are engaged and are used as a means like the sending down of the Quran; thus, the angels who are used as a means are indicated when the word we is.

For instance, when Allah says,"We have, without doubt, sent down the Message", the verse is understood as follows: "I sent down the Quran to my beloved Prophet through my angels. When Allah says, “We send down pure water from the sky”, attention is attracted to the angel in charge of bringing rain drops down; the angel is also included in this expression.
However, these angels have no effect on these deeds. For, the real doer of deeds is Allah. 

3- In the verses in which creation is narrated, Allah usually uses the word I. For instance, “I have only created jinn and men, that they may serve Me.” (adh-Dhariyat: 56)
However, in some verses in which creation is narrated, the word "We" is used. What is meant by "We" is the fact that the angels witnessed the deed of creation and that they were there. With this expression, Allah makes angels witness His words.

For instance, when Allah says, “We created not the heavens and the earth and all between them” (al-Ahqaf: 3), it is understood that He created them alone but the angels were there and that they witnessed this creation.

4- With this expression, Allah teaches us something and tells us, "Say we instead of I; do not display egoism; do not boast; keep away from conceitedness; rise with modesty; be the perfect man by getting rid of your ego.”