The primary reason is that Jessica has fallen in love with Lorenzo, who is a Christian. She knows that her father will never allow her to marry a Christian because he hates them with a passion. Also, Jessica is ashamed of her father. She states in Act Two, Scene Three:
Alack, what heinous sin is it in me
To be ashamed to be my father's child!
She also does not share her father's style of doing things nor his attitude. As she further states:
But though I am a daughter to his blood,
I am not to his manners.
Shylock is known to be extremely stingy and materialistic. He cares more about his possessions than anything else. He is in the business of making money through loan-sharking and has made a name for himself as a formidably shrewd and demanding moneylender.
His stinginess is also made evident in his neglect of Launcelot, his manservant, whom he does not feed, pay, or dress well. Launcelot also decides to leave as a result. Jessica evidently does not share her father's parsimony or his hatred, and believes that Lorenzo will rescue her from her father's imposed misery.
If thou keep promise, I shall end this strife,
Become a Christian and thy loving wife.
She is prepared to give up everything she has learned and lived up to now. She has had enough of her father, his strictness, and his greed. After marrying Lorenzo she states:
I shall be saved by my husband; he hath made me a
The differences between her and her father are also emphasized by Salarino when he, after Jessica's disappearance, tells Shylock:
There is more difference between thy flesh and hers
than between jet and ivory; more between your bloods
than there is between red wine and rhenish.
Jessica takes her father's money probably as compensation for the life of misery she feels he has put her through. Furthermore, she needs money to fund her and Lorenzo's escape. The two plan to go to Belmont, Portia's home, to get married. They will also need money to sustain themselves.
Moreover, Jessica takes as much as she can so that she can have fun. It is easy to assume that her father has never spent much on her because he is so stingy. Having all this money gives her an opportunity to indulge almost her every wish. This becomes apparent when one reads about how she spent Shylock's precious possessions.
Shylock has heard what a spendthrift Jessica has been and despairingly declares:
Why, there, there, there, there! a diamond gone,
cost me two thousand ducats in Frankfort! The curse
never fell upon our nation till now; I never felt it
till now: two thousand ducats in that; and other
precious, precious jewels.
Tubal also tells him:
Your daughter spent in Genoa, as I heard, in one
night fourscore ducats.
Jessica is clearly having a good time, because she also bought a monkey by paying with one of her father's most treasured possessions. As Tubal reports:
One of them showed me a ring that he had of your
daughter for a monkey.
The end result is that Shylock vehemently denounces his daughter for her betrayal and wishes her dead. He states that he prefers seeing his possessions returned in a coffin containing her dead corpse than to see her alive.
I would my daughter
were dead at my foot, and the jewels in her ear!
would she were hearsed at my foot, and the ducats in
This, on its own, makes him a most despicable character.