If you are talking of The Merchant Of Venice then the answer is:

1. Both Bassanio and Antonio are very good friends. Antonio understands the problem of his friend and had a desire to help him. So he lends him money because he know that Bassanio is short of money at that time and he can't help himself.

2. Antonio knows that his ships are coming from the sea carrying a lot of money and he will surely repay the loan after the the ships reach him.

3. Antonio don't want his best friend to break friendship with him only for some money.