This is especially true at 4-way stops where each person goes through in the order they arrived at the intersection. If you arrive first but attempt to let someone else go through out of courtesy you may confuse other drivers including the person you're trying to let through, leading to wasted time and a potential accident.
There are notable exceptions however, such as letting someone merge by moving over a lane or slowing to allow someone into your lane. Be nice, but follow the rules everyone else does. They exist for a reason.