User blog comment:JediSpectre117/Citadel DLC, conspiracy, Thoughts?/@comment-1480659-20130225233044/@comment-4121017-20130226185924

Mainly when Shepard asks about how Synthesis works. The Catalyst said it tried many options in the past, and all of them have failed until Shepard had came along. Thanks to the completion of the Crucible, s/he convinced it that its solution won't work anymore and left the options in their hands. If Shepard decided to go through with activating the Crucible, the Catalyst would be convinced that its wrong in its belief of synthetics and organics destroying each other.