It was jealousy and resentment, plain and simple! And man's failure to trust in YHWH (Yahweh)! Take a look at how things unfolded:
In Genesis 15 we see our first Patriarch Abraham complaining to YHWH that he had no heirs to pass an inheritance on to.
Genesis 15: 1 Some time later the word of ADONAI came to Avram in a vision: "Don't be afraid, Avram. I am your protector; your reward will be very great." 2 Avram replied, "ADONAI, God, what good will your gifts be to me if I continue childless; and Eli'ezer from Dammesek inherits my possessions? 3 You haven't given me a child," Avram continued, "so someone born in my house will be my heir." 4 But the word of ADONAI came to him: "This man will not be your heir. No, your heir will be a child from your own body." 5 Then he brought him outside and said, "Look up at the sky, and count the stars - if you can count them! Your descendants will be that many!" 6 He believed in ADONAI, and he credited it to him as righteousness.
Now, some time passed and in Genesis 16 we see Abraham and his wife Sarah panicking about not having an heir yet, and so they decided (at the urging of Sarah) to take matters into their own hands and have Abraham sleep with his wife's pagan/Egyptian servant Hagar. The result of that union was a son they named Ishmael.
In Genesis 16:13-16 the Angel of the YHWH told Hagar (who resented the fact she had conceived) that her son Ishmael would live and end up being "...a wild donkey of a man, His hand will be against everyone, and everyone's hand will be against him; and he will live to the east of all his brothers."
Ishmael - the son of the Egyptian slave woman - is the father of the Arab nations, most of whom today are Muslim....
When Abraham (then still known as Avram before God changed His Name) was 99 years old, YHWH appeared and made a covenant with him, promising to make Avraham the father of many nations - which meant at some point he would have another heir and that that other heir, and not Ishmael, would be the one with whom YHWH would make his next covenant:
Genesis 17: 1 When Avram was 99 years old ADONAI appeared to Avram and said to him, "I am El Shaddai [God Almighty]. Walk in my presence and be pure-hearted. 2 I will make my covenant between me and you, and I will increase your numbers greatly." 3 Avram fell on his face, and God continued speaking with him: 4 "As for me, this is my covenant with you: you will be the father of many nations. 5 Your name will no longer be Avram [exalted father], but your name will be Avraham [father of many], because I have made you the father of many nations. 6 I will cause you to be very fruitful. I will make nations of you, kings will descend from you.
7 "I am establishing my covenant between me and you, along with your descendants after you, generation after generation, as an everlasting covenant, to be God for you and for your descendants after you. 8 I will give you and your descendants after you the land in which you are now foreigners, all the land of Kena'an, as a permanent possession; and I will be their God."
Genesis 17: 15 God said to Avraham, "As for Sarai your wife, you are not to call her Sarai [mockery]; her name is to be Sarah [princess]. 16 I will bless her; moreover, I will give you a son by her. Truly I will bless her: she will be a mother of nations; kings of peoples will come from her (this included Yeshua!!!)." 17 At this Avraham fell on his face and laughed - he thought to himself, "Will a child be born to a man a hundred years old? Will Sarah give birth at ninety?" 18 Avraham said to God, "If only Yishma'el could live in your presence!"
19 God answered, "No, but Sarah your wife will bear you a son, and you are to call him Yitz'chak (Isaac)[laughter]. I will establish my covenant with him as an everlasting covenant for his descendants after him. 20 But as for Yishma'el, I have heard you. I have blessed him. I will make him fruitful and give him many descendants. He will father twelve princes, and I will make him a great nation. 21 But I will establish my covenant with Yitz'chak, whom Sarah will bear to you at this time next year." 22 With that, God finished speaking with Avraham and went up from him.
23 Avraham took Yishma'el his son, all the slaves born in his house and all who had been bought with his money, every male among the people in Avraham's household, and circumcised the flesh of their foreskin that very day, just as God had said to him. 24 Avraham was ninety-nine years old when he was circumcised in the flesh of his foreskin, 25 and Yishma'el his son was thirteen years old when he was circumcised in the flesh of his foreskin.