Usopp was greatly offended by Luffy's decision for various reasons. The ship was a gift from his childhood friend Kaya, which meant a lot to him. However, Usopp's inferiority complex and fear of being left behind for being weak pushed him to disagree with Luffy. This resulted in Usopp temporarily leaving the crew

Luffy: "I- It's too much. . ." he sobs, hand on his hat

Zoro: "Don't lose yourself. That's what a captain is! Don't hesitate. Stand by yourself. If you waver, who can we trust?"