The friend had a very boring and sad life. It seems that they were pushing him into a relationship that he didn't want, and he likely didn't get promoted because he wasn't interested in her. He got frustrated and disillusioned, and he decided to rob the bank. Mostly because he wasn't getting promoted or making much money, but he was handling a ton of cash and making the bank a lot of money. He regretted it at the last second, but it was too late by that point.
