If a thing has a definite shape, it's clear that it's countable. I guess every student can grasp this without difficulty.
But when a word's meanings are somewhat abstract, things become tricky.
change, shape, difficulty, disease, illness, construction, illusion, confusion..Words like these are all countable yet uncountable.
I doubt anyone can explain very successfully the reason why change is taken as countable under certain circumstances while uncountable under other circumstances. It's always been a challenge to my own construction of sentences.
But, of course, Englishforums is a wonderful website, maybe someone CAN explain this stuff. I don't know...
Regards!
Maple