Here's why:
* -ful is a suffix that means "full of" or "characterized by".
* Prideful means "full of pride".
The other words have different suffixes:
* dispatch has the suffix "-ch".
* relate has the suffix "-ate".
* untie has the prefix "un-".
Here's why:
* -ful is a suffix that means "full of" or "characterized by".
* Prideful means "full of pride".
The other words have different suffixes:
* dispatch has the suffix "-ch".
* relate has the suffix "-ate".
* untie has the prefix "un-".