Grendel is utterly infuriated by the sweet melodies and joyful laughter that regularly emanate from the Danes' mead-hall, Heorot. Each and every day, as he lurks in darkness in his lair, Grendel is assailed by “light-hearted laughter” coming loudly from the hall, “dulcet harp-music”, and the “clear song of the singer.”

One such song recounts the creation of the earth, which only serves to antagonize the monster even further. As a descendant of Cain— Adam's son who murdered his brother Abel—he feels estranged from the God worshipped by the Danes. Not unreasonably, he doesn't wish to be reminded of God's creation of the earth and, by extension, of Cain's eventual exile.

The joy that inspires the sweet melodies emanating from Heorot is notable by its absence from Grendel's miserable existence. Living in both literal and figurative darkness, he represents everything that is evil, monstrous, and perverted. There is no music in his life, no conviviality or enjoyment. Just plenty of brooding, resentment, and anger, which build up over time as the Danes celebrate in their mead-hall.

Grendel instinctively rebels, then, against the Danes, their music, and their laughter. Unable to stand any more of their merry-making, he sets out to bring terror and chaos to Heorot.

Your questions refers to lines 3 - 13 of Beowulf--this section describes the history of Scyld the Scefing, leader of the Spear-Danes and Beowulf's father. While this section would have been sung by a scop, the section describing Heorot and the songs sung by the bards in that mead hall does not begin until line 25: "The greatest of hall-buildings; Heorot, he named it." As such, I will focus on this section (ll. 25 - 62) which describes the establishment of Heorot, the "high and horn-crested hall" with its "dulcet harp music/Clear song of the singer." Indeed this section presents us with a direct contrast between the emotions to be heard in the hall, and the life Grendel leads, as he is described as being "bitter" at having to hear this "light-hearted laughter" on a daily basis. The nature of the singing and the jovial atmosphere of the warriors in Heorot makes Grendel feel increasingly embittered about his own situation as an outcast. 

Grendel in this poem fits an archetype seen, in various forms, throughout much Anglo-Saxon poetry: that of the outsider, as in the poem The Wanderer--someone who is forced, because of some social transgression, to live beyond the borders of society, excluded from its joys. It is usually believed that Beowulf is a story that pre-dates Christianity among the Anglo-Saxons, and to which amendments have been made to wed Christian concepts to heroic concepts. This can be seen here, in that the songs sung in Heorot often feature Bible stories, descriptions of "earthmen's beginnings," the "Father Almighty" and a world lightened by the sun and moon. This stands in direct contrast to the situation in which Grendel finds himself. Grendel, far from being able to enjoy himself in a hall with his companions, is alone; "he who bided in darkness" does not experience the touch of the sun's beams. Moreover, as the son of Cain, we hear that Grendel has been explicitly cast out by God and forced to wander the moors and fens alone. Anglo-Saxon society feared little so much as being exiled; it is a constant preoccupation in the surviving Anglo-Saxon writings. As such, we can see in this section that Grendel is demarcated very distinctly as living a life that is the sum of the Spear-Danes' fears. 

The bard in Heorot, Hrothgar's hall, sings about God creating the earth. The bard's song is about the way in which God made the earth and the fields and the sun and the moon (to create light for humans). God also created flora and fauna. The Spear Danes celebrate God's power in their communal songs. Grendel, on the other hand, lives a life of seclusion and loneliness on the moors, and he is enraged by the songs he hears in Heorot. Grendel is the descendant of Cain, who God banished (along with his descendants) from the company of other humans after Cain murdered Abel. Unlike the people in Heorot, who celebrate God's creation and glory in a communal way, Grendel lives a life of misery on the moors and spends his life fighting God and God's people. 

This is a great question because it addresses two primary components of Anglo-Saxon philosophy: the battle between good and evil, and the role of God in the conquests of the armies.

Christianity was a new idea when this poem was composed, and this poem intertwines Christian concepts of good and evil with ancient Scandinavian mythology and lore.

Grendel, called a descendent of Cain, is a monstrous personification of evil. Christianity teaches that evil (darkness) cannot stand goodness (light.) When the soldiers sang their joyful triumph songs, Grendel's nature compelled him to destroy their happiness and celebration.

Heorot was a place of light (the whole hall was covered inside and out with gold), while Grendel dwelt in darkness. The scop tells that the the joy and light are from the Almighty, while Grendel's hellish abode reeks of fiends and goblins and monsters that forever oppose the will of the Lord.