During their conversation in Act One, Scene One, Bassanio tells Antonio:
... but my chief care
Is to come fairly off from the great debts
Wherein my time something too prodigal
Hath left me gaged.
.... from your love I have a warranty
To unburden all my plots and purposes
How to get clear of all the debts I owe.
The first reason which Bassanio provides here is to clear all his debts. He has been too busy and neglectful to see to them being paid on time. He also expresses his faith in Antonio's love for him as a guarantee to help him pay such debts.
Antonio assures him that all his fortunes are at Bassanio's disposal and Bassanio admits that he is unable to repay a debt he already owes Antonio, but rest assured, he will sttle both the first debt and the one he is about to make. He uses an apt metaphor of using a second arrow flying in the same direction as the first which was lost, to regain it.
The good Antonio tells him not to be too concerned and that he should tell him what he needs.
The second reason that Bassanio provides is expressed as follows:
In Belmont is a lady richly left;
And she is fair, and, fairer than that word,
Of wondrous virtues: sometimes from her eyes
I did receive fair speechless messages:
Her name is Portia, ...
... many Jasons come in quest of her.
O my Antonio, had I but the means
To hold a rival place with one of them,
I have a mind presages me such thrift,
That I should questionless be fortunate!
Bassanio states that he wishes to woo the beautiful and wealthy Portia from Belmont. She had shown an interest in him and he wants to compete against the many suitors who had come from far and wide to win her hand. He feels that he cannot rival them since he does not have the means (in financial terms) to do so. He believes that he will, if he should be able to do so (if he has money), be so thrifty that he will stand an equal chance to win her over. He therefore needs Antonio's help to give him a financial boost.
The magnanimous Antonio promises to help his desperate friend, but states that he has no cash on hand or goods as security to raise funds, because of his mercantile ventures. He asks Bassanio to venture to Venice and use his good name to obtain a loan. He shall, on his part, do the same so that Bassanio may have the means to go to Belmont and there win Portia's hand.