Continuity mistake: When Sharkboy is feeding the sharks on the dock at the beginning, there is a yellow canister behind him which disappears then reappears. (00:01:15)
Continuity mistake: When Max and his Mother walk down their garden path on his way to school, between a wideshot and closeup the position of his mothers hair completely changes. (00:13:50)
Continuity mistake: When Minus has the heroes captured and hanging down from wires, there is a shot where he is threatening Max and Sharkboy is nowhere to be seen. There is no wire either, so it's not like he's out of sight.
Answer: According to Taylor Lautner, the actor who played Sharkboy, he approached the role as if Sharkboy was jealous of Max because Sharkboy has a crush on Lavagirl and she pays Max lots of attention.
BaconIsMyBFF