Estella knows the strength of Pip's feelings for her, but she wants him to know that Miss Havisham has raised her to break the hearts of men and, thus, she will never be able to return his love. Her statement is actually a contradiction of her true character. Because Estella does have a heart, softness, and sympathy, she issues this warning to Pip. She genuinely cares about him and does not want him to continue on his futile quest for her affection. She is rebelling against Miss Havisham's wishes, for in telling Pip this, Estella demonstrates that she is unwilling to break every man's heart.
Her statement not only serves as a warning to Pip; it also foreshadows her choice of an abusive husband and an unhappy marriage.