N. M Gwynne has proven that happiness depends on grammar. No, really. Here he is in Chapter Two of Gwynne’s Grammar, which would make a nice little stocking stuffer for the language lover on your list:
Step one. For genuine thinking, we need words. (By “genuine thinking” I mean as opposed to merely being conscious of feeling hungry, tired, angry and so on and wanting to do something about it; in other words, anything that animals cannot do.) Thinking cannot be done without words.
Step two. If we do night use words rightly, we shall not think rightly.
Step three. If we do not think rightly, we cannot reliably decide rightly, because good decisions depend on accurate thinking.
Step four. If we do not decide rightly, we shall make a mess of our lives and also of other people’s lives to the extent that we have an influence on other people.
Step five. If we make a mess of our lives, we shall make ourselves and other people unhappy.
In summary of the proof: grammar is the science of using words rightly, leading to thinking rightly, leading to deciding rightly, without which—as both common sense and experience show—happiness is impossible. Therefore, happiness depends at least partly in good grammar.