Board Thread:General Discussion/@comment-32182236-20170602170443/@comment-32182236-20170804133513

Monster SOULs are supposedly made of love, hope, and compassion. It would be these three things Asriel lost when he became Flowey. The monster body is made of magic. Asriel tried to feel love from Asgore and Toriel, to no avail.

Perhaps Asriel losing these feelings when he became Flowey would be why monsters assumed SOULS were made of these things. If losing your SOUL means losing those three things, it's easy to jump to the conclusion that your SOUL is those three things. The SOUL likely only contains the ABILITY to have those three things, though, hence why humans don't always have them.