Our Sages explain that, in contrast to other living beings which were created in pairs, man was created alone.

Why?

So that every individual should say, "The world was created for me," and thus appreciate that his conduct can affect the totality of existence.

The coming of the Redemption depends on every single individual. Simply put, were people to open their eyes, the door would open and Moshiach would enter.(The Rebbe, 7 Tevet, 5752)
(L'Chaim)