A guy is in love with a girl. We understand that situation and know that people in love will tolerate a lot from each other. But if you take a businessman who is so pressed for time that he hasn't time to even see what is happening around him, and you have him suddenly decide to help a poor guy on the street, no one is going to buy that because most people won't help others on the street. That has to be shown. You have to start with a scene two years earlier when he or his brother were the person on the street and someone helped them.
Many motives are normal typical human motives that everyone has.
They don't really need to be shown. Love is an example. Sentiment for a poor orphan girl - that grabs everyone. Caring for a baby. Parents looking out for the welfare of their children.