What's the "gari" in 恥ずかしがり屋?

Presumably it's some kind of かりる.... Per Jisho, there's a 恥ずかしがる; not seeing why that's necessary, since 恥ずかしい should act as a verb on its own.... What would be the difference between 恥ずかしいです and 恥ずかしがります?

Isn't there a descriptive suffix that's like that がる, for expressing how other people seem to feel? Maybe that's it.

Here's what someone online says about -がる:

It is a derivational morpheme that makes a verb when suffixed to the adnominal form of an adjective. Adjectives used with -garu express emotion (kowa-garu, sumana-garu) cognition (mezurashi-garu, abuna-garu) and desire (hoshi-garu).

Interesting: this thread says that がる has specific connotations of being [adjective] to such an extent that the effect is visible to others. So, maybe, 妹は車が欲しいだろう but 妹は車を欲しがっている—with を. 正しいかなぁ。 So, someone weeping and rending his/her garments might 悲しが（ってい)る, I guess, and the sort of person who does that a lot might be a 悲しがりや. I suppose that would be different from someone who is, 例えば, 強い or 厳しい.

Not at all clear on what the passage above means with cognition. どんなことが珍しがるかなぁ。

Off to read 元気 chapter 20!