Please translate this line to plain English?

Question

"And yet I know not how conceit may rob the treasury of life when life itself yields to the theft."

Funny answers welcomed but I won't vote for you. Unless it's really funny, of course.

Answer 1

It means arrogance and vanity isn't uninvited.  The poem puts it this way: you can't rob something that you're welcome to take.

Answer 2

It's about self destruction. People know being conceited will make their life less rich (they will lose friends, respect, cause friction) yet still become conceited.