Being assertive isn't the same as being reckless. Assertiveness doesn't mean being a lone-wolf designer. Asking questions or getting a second opinion isn't in conflict with being assertive. Having an opinion about a feature or a design decision is necessary, but validating that assumption or opinion with people you trust isn't a sign of weakness.
People who ask for advice in order to refine their prior thinking or opinions are more trustworthy designers. Those designers don't assume themselves to be perfect and recognize that intuition isn't always the best way to navigate a design process.