Is it considered a sin or a mistake to sometimes not stop to help those in need on the road, especially when one's wife is present, or to not always stop to help potential colleagues when going to work?

There is no sin upon one who passes people in the street and does not help them, unless he knows of their dire need and it becomes obligatory upon him to help them. Many texts have been revealed that encourage helping those in need, among them the saying of the Messenger of Allah, peace and blessings be upon him: "Whoever has a surplus mount should give it to one who has no mount, and whoever has a surplus of provisions should give it to one who has no provisions." This assistance is not obligatory, and refraining from it does not incur sin, but one misses out on the virtue and reward.