Because her father refused to accept the relationship and marriage of her daughter that's why she defy with father and elope with her lover

Reasons for Defying Father and Eloping

The first stanza of the poem "Lord Ullin's Daughter" by Thomas Campbell sheds light on the reasons why Lord Ullin's daughter defies her father and elopes with her lover. Here are the details:

Love and Desire
Lord Ullin's daughter is deeply in love with her lover, who is described as a "youth of low degree." This intense love and desire for her lover drive her to defy her father's wishes and elope with him. The passion and longing she feels for her lover overshadow any concerns she may have about her father's disapproval.

Disapproval and Opposition
Lord Ullin, being a noble and powerful figure, likely disapproves of his daughter's relationship with a man of "low degree." His opposition to their love may stem from concerns about social status, reputation, or the well-being of his daughter. Despite her father's disapproval, Lord Ullin's daughter is determined to be with her lover, even if it means defying her father's authority.

Romantic Idealism
The act of eloping with her lover may also be driven by a sense of romantic idealism on the part of Lord Ullin's daughter. She may believe in the power of love to conquer all obstacles and be willing to risk everything for the sake of love and happiness. This romantic idealism fuels her decision to defy her father and follow her heart.

In conclusion, Lord Ullin's daughter defies her father and elopes with her lover due to her deep love and desire for him, her defiance of her father's disapproval, and her belief in the power of romantic idealism. These factors combined lead her to make the fateful decision to leave her father's home and embark on a risky journey with her lover.