The thing to know about Bassanio is that he loves his lavish lifestyle, but he's really bad with money, which is why he ends up borrowing from Shylock. When we meet Bassanio, one of the first things out of his mouth is:
Tis not unknown to you, Antonio,
How much I have disabled mine estate
By something showing a more swelling port
Than my faint means would grant continuance (1.1.4)
Translation: "Everyone knows I live way beyond my means and have blown all my money." Not only that, but Bassanio's spending has buried him under a big pile of debt and he's hoping to pay it all off. When he says "To you, Antonio, / I owe the most in money and in love" (1.1.4), it becomes pretty clear that Bassanio has been sponging off his rich BFF. And even though Bassanio says he loves Antonio, Bassanio looks, sounds, and smells like a big user, the kind of guy willing to take as much as his generous friend has to offer. (The 90s R&B girl group TLC had a name for this kind of guy – Bassanio is a classic "scrub.")
So how does Bassanio plan to get out of debt? By borrowing even more money so he can hook up with a rich heiress, Portia, who will pay off all his loans and continue to float his rap-star lifestyle. He's even willing to let his best friend risk his life by putting up a "pound of flesh" as collateral so he can take out a personal loan from Shylock. What kind of a person does that?
If you thought Bassanio was bad for using Antonio, check out how he talks about Portia: "In Belmont is a lady richly left, / And she is fair [...] Her name is Portia, nothing undervalued [...] Nor is the wide world ignorant of her worth" (1.1.6). Hmm. Notice how Bassanio uses words like "value" and "worth" to describe his future wife? It's clear that Bassanio sees Portia as a meal ticket.
We're not saying Bassanio doesn't care about Portia. There does seem to be some real affection between the two when they're hanging out in Belmont. What we are saying is this: even after he gets hitched to Portia, Bassanio's loyalty to his new wife is questionable and he seems to value his bromance with Antonio more than anything else. Our evidence? Well, Bassanio admits that Antonio is his number one priority when he rushes from Portia's house to Antonio's trial. "Antonio," he says. "I am married to a wife / Which is as dear to me as life itself, But life itself, my wife, and all the world / Are not with me esteemed above thy life" (4.1.8).
Bassanio seems to float through life, reaping the benefits of his rich friends. In the end, Bassanio gets everything he ever wanted: he snags a rich wife who is devoted to his happiness and his best friend is saved from Shylock's vengeful lawsuit.