it+is+necessary+that+we+be+exposed+为什么用被动？

动词expose为及物动词，当其宾语位于它之前时，要用被动语态。又如：

The government require the Chinese students to learn English as a second language.

The Chinese students are required to learn English as a second language.

注：以上两句句意完全相同，第一句是主动语态，第二句是被动语态。