This was the case for all the girls, really- Monika amplified their struggles till they were too much to cope with.
The writer stated before that Sayori just had depression some times you can just get for no reason and because you tell yourself you shouldn't have depression so you should be fine it becomes ridiculously hard to deal with especially when you're known to be the up-beat one. That was what Sayorinwas going through before Monika cranked it up.