Full Idea
A multiverse is inevitable if inflation is eternal, and eternal inflation is by far the most likely possibility in most, if not all, inflationary scenarios.
Gist of Idea
It seems likely that cosmic inflation is eternal, and this would make a multiverse inevitable
Source
Lawrence M. Krauss (A Universe from Nothing [2012], 08)
Book Reference
Krauss,Lawrence M.: 'A Universe from Nothing' [Simon and Schuster 2012], p.129