This may also explain why we are not usually sensitive to our own smells —we simply do not need to be.
主语是This,本句是复合句.主干This may also explain…后面的宾语部分为why引导的宾语从句充当,破折号后为原因的具体内容,be后省略了sensitive to our own smells.
翻译：这也可以解释为什么我们通常对自身的各种气味不敏感——只不过我们不需要这样做.