The Going Merry was pretty much Usopp's treasure, he got it from Kaya when he left, and he's pretty much protected it (as much as he can, along with the rest of the crew) and repaired it to keep it going. So when Luffy made that decision, whether it was captain's orders or not, you just new Usopp was going to have something to say, and more then likely (like he did) fight for it. He was fighting for his treasure, even if that meant fighting his captain. He didn't care, he wasn't thinking clearly, all he could think was that he had to protect it. And he tried so hard, and I will always love him for that....