〜がる has to do with expressing emotions, making it clear how one feels. It is a 五段 Auxiliary verb. It goes with a limited set of adjectives (List of words that can go with がる). It has special cases for 〜がり and 〜がり屋
Main interpretations
it has 2 main uses:
Expressing emotion (very clearly) to the outside world
One use is if the subject really expresses some emotion to the outside world, or they are making it extremely clear.
アイダは日本での生活を懐かしがっています。 Aida misses her time of living of japan (she's really acting like it, making it very obvious).
Positively stating someone else's emotions
You cannot use the positive form to state someone else's emotions. Using 〜がる does allow for this. One way this can be interpreted is by comparing it to the first rule: You cannot positively state someone else's emotions directly, because you do not know how they feel. You can say that the look like they are having an emotion though, but only know if they making it clear externally.
闇が怖い I am afraid of the dark
子供も大人も暗闇を~~怖い~~のはとても一般的です 子供も大人も暗闇を怖がるのはとても一般的です it is common for both adults and children to be afraid of the deep dark (they appear to be afraid)
Note that no such rule about not being able to state someone's emotion for anything that's not a positive though. The following sentence is valid:
狼が怖くないけれど Wolves are not afraid, however.
Standing out (from from crowd)
The speaker is having themselves stand out from everyone around them.
俺おれだけが面白がっていたのか？ Am I the only one who thought it was amusing?
Nuances
Feelings do not have to be as expected
The internal state and perceived state of the subject of a ～がる do not have to match. The feelings can be uncharacteristic, or contrast what we know/think of them. Normal is also fine though.
彼は何も言わなかったが、心の中では{悔|くや}しがっていた。 He did not say anything (he looked alright with it) but, deep inside he was very disappointed.
Eyeroll
がる sometimes alludes to a bit of a critical tone, bit of a 'pfffft look at this guy'. This is on top of the two main meanings.
あの{小僧|こぞう}、2匹めで色違いのポケモンが出て嬉しがっていた。 pfft, that little prat, being all pleased when the second pokemon that came out was a shiny.
誰でも神を信じたがります。 Everyone wants to believe in god (what fools they are, the speaker thinks)
Grouchy
When used with negative emotions, the speaker sounds grumpy.
私は今苦しがっています。 I'm suffering (in case you didn't know)
私は子供のころ、歯医者に行くのを嫌がっていました。 When I was a kid, I hated it when I went to a dentist.
Transitivity
〜がる is generally intransitive, but if a こと・の is added it can be transitive.
お父さんに会ったけど、すごく寂しがっていたよ。 I met my dad, but he seemed really lonely. (intr.)
彼は東京に帰れない==ことを悲しがっていた==。 He seemed sad about not being able to return Tokyo. (trans.)
List of words that can go with がる
Common
90% of the times 〜がる is used, it's this.
- 〜たがる: To (seem) eager to
- 欲しがる
- 寂しがる
- 怖がる
- 嫌がる
- 可愛がる: To treat tenderly
- 不安がる
- 面白がる: To be amused
- 嬉しがる
- 痛がる
- 悲しがる
- 強がる: Usually to put on a brave front
- 煙たがる:
- To consider (...) a nuisance
- to be sensitive to smoke
- 恐ろしがる
- 羨ましがる
- 訝しがる: to doubt
- 残念がる
- 悔しがる
- うるさがる: to feel annoyed at
- {有難|ありがた}: to be thankful for
- 気の毒がる: to (seem to) pity
Uncommon
- きつがる
- 辛がる: To (seem to be) in emotional pain
- 苦しがる: To complain of pain
- 重たがる: To make out to be heavy
- 新しがる: to like new things
- 暑がる
- 寒がる
- 珍しがる: to think (...) a curiosity
- 汚がる: to make (...) out to be dirty
- 不思議がる
- 不愉快がる
- 不快がる
- 不気味がる
- 心細がる
- 気持悪がる
- 悪がる
- 興がる (rare form attached to noun, only written language)
- 煙がる
- 恋しがる
- すまながる
- 不憫がる
- 悩ましがる
Very rare
- {偉|えら}がる: to think highly of oneself
- 得意がる: To be inflated with pride
- 意外がる: To feel (something is) unexpected
- 古がる: to be fond of old things
- 涼しがる
- 決まり悪がる: to seem ashamed
- 気味悪がる
- 楽しがる
- 哀れがる (rare form attached to noun, only written language)
- 憎がる
- 良がる: to feel satisfied
- 気持ちよがる
- 弱がる: to feign being weak
- 独りよがる
- 憎たらしがる
- 妬ましがる
- 妬がる
- 煩わしがる
- 好もしがる: To outwardly prefer