Board Thread:General Discussion/@comment-28128661-20160404143233/@comment-27907368-20160406013031

It's times like this that I wish we knew more about the DT Extraction. All we are given is: ... I wonder what blueprints she had? Were they like the ones sans had? Hm. Anyway, I believe Alphys extracted DE-TERMINATION from the human souls which made them weak enough to shatter at the end of neutral runs.
 * ENTRY NUMBER 5: I've done it. Using the blueprints, I've extracted it from the human SOULs. I believe this is what gives their SOULs the strength to persist after death. The will to keep living... The resolve to change fate. Let's call this power... "Determination."

Seeing as the DT Extraction occurs shortly before Flowey is created, it could be that Flowey is able to house more DETERMINATION than the amalgamates and the (now weakened) SOULS because he has a more physical form that came from the Surface. He then can reset. Monsters turn to smoothies because they aren't physical enough to handle it.

Oh man, explaining why the weakened SOULs trump our DETERMINATION in a neutral run will depend a lot on how you interpret the story. Since I believe that we play as an amnesiac puppeteering Frisk's body, it's no reach to say that we are at our weakest in neutral runs. Neither Frisk nor 's DETERMINATION has been sparked by their respective endings, so they're kind of just cruising.