How come you are so interested in the environment?

这里的come是系动词,相当天be动词或become,come link-v。成为、变为、变得。you are so interested in the environment是省略了that的表语从句。how come 后面直接加表语从句（不用倒装）,表达一种语气：愤怒或惊讶。美口语how come ：怎么会（是）。…（这样）,这是美式英语口语的固定搭配。