I believe it us hugely about personalities and dealing with development of the child.
Each kid and each parent is different and their individual relationship. Some kids really struggle with tantrums and the like others don't. Some parents deal with tantrums with ease and others don't.
2, 3 and 4 are huge developmently years. You learn so much more and lots of developmental and behavioral issuses show up at those ages.
Anyone who puts a blanket 'its for just one reason' is a freaking fool.