Why is the United States frustrated with European involvement in Ukraine's defense?

The United States is growing frustrated with European reluctance to take a leading role in Ukraine's defense despite Ukraine being geographically central to Europe, bordering countries like Hungary, Poland, and Romania. While Europeans have increased their defense spending, they remain hesitant to take further actions and prefer the US to foot the bill for security challenges on their own continent. The Trump administration has been particularly vocal about this issue, emphasizing that European nations should step up to the table, increase their defense spending, and take responsibility for regional security matters. As Ben Shapiro notes, this approach of pressuring Europeans to fulfill their obligations is 'not a stupid tactic' but often 'a smart tactic' in international relations.