You know, it's implied that the other humans could also reset; but Asgore, without his spirit broken, was strong enough that they fought him over and over until they each gave up.
It's also commonly speculated that the yellow soul was the last one before you.
Think about that. After all he's done, Asgore is trying desperately to remind himself why he's doing this, to hold himself together; and the thing that finally breaks his conviction is when one of the children, tears in his eyes, agrees with him. The child agrees completely that humanity's past actions are their own fault. Because hearing that, you can't help but understand how horrible all this is.
The soul is taken, of course. Yellow doesn't make it hard; he's already given in. But Asgore has lost that fighting spirit that overcame the determination of those before you.