がる

「がる」is used to express emotions, feelings, desires that are shown through the facial expressions and actions of others, not oneself.
Because the emotions, feelings, and desires are observed through the facial expressions and actions,「がる」sometimes carries the meaning: the person being talked about appears to be that way, but it may not be what they truly feel. This meaning needs to be understood based on the context.
Notes:
- This structure is essentially a verbification of adjectives, so the particle が goes with the adjective. When used with adjectives like 「ほしい」or「たい」, it changes to を when used with「がる」.
- There are some adjectives that do not describe emotions or feelings but still go with「がる」such as: 新しがる、重宝がる、強がる、粋がる...
- Comparison between「がる」and「がっている」:「がる」expresses emotions, feelings that frequently happen or have become a habit.「がっている」expresses temporary emotions, feelings happening at the moment.
-「がる」is primarily used for third persons, but the speaker can also use it for themselves, when objectively looking at themselves from a third person's perspective. For example:
父が亡くなったのは辛いけど、いつまでも悲しがっているわけにはいかない。 (It's hard that my father passed away, but I can't keep being sad forever.)