Slovak Noun Declension

[Daniel N.]

It's actually the opposite. Proto-Slavic had many declensions, weird things from today point of view. The lesser used words got more regular over time: who cares about a word you use once a month? The most common words retained the complexity, since you hear the pattern over and over when you're a child.

That's why personal pronouns and the verb to be are "irregular" (i.e. have a very specific pattern) in almost all Indo-European languages

[Cavesa]

Thanks, I get no problem believing the opposite is true.
But the main point still stands: the more common words can be much more discouraging than the less common ones.

[Daniel N.]

I agree, but there's usually a small number of them. How many verbs does English have? 10000? more? 95% of them are regular - likely more - and even if there's an irregular verb you'll use once a year, who cares - likely many native speakers regularize it as well.

The most regular things are repeated so often that their sometimes unusual pattern sticks to memory.

But there's another point I keep repeating: languages are - for some unknown reason - needlessly complex. We don't really need irregular verbs and nouns, all those patterns. If all verbs and nouns were "regular" (i.e. only one pattern), we wouldn't feel any loss: it wouldn't be like losing a tense or case.