Antonio's chiefly involved in Bassanio's romance by way of providing him with money. In this status-conscious age, it's important to have either wealth or position to marry an aristocratic lady like Portia. As Bassanio has precious little of either, he instinctively turns to his good friend Antonio to help him out. Bassanio has often borrowed money from Antonio in the past, and as Antonio's such a soft touch, he has no hesitation in lending him some more. Bassanio's a young man in love, a hopeless romantic with a somewhat naive view of life. But he's still all too aware of the harsh realities of finding a wife in his hierarchical society; if he wants to woo Portia, he's going to need to get his hands on some ready cash.
Though Portia is very much in love with Bassanio, she also knows that it would be considered beneath the dignity of her class to marry someone without means. Love is important to Portia, but society's conventions must also be given their due. So Bassanio has little choice but to borrow money from Antonio; the prevailing social standards demand it.