Tategaki vs Yokogaki
Japanese can be written horizontally as well as vertically. Traditionally, vertical writing (縦書き tategaki) was the standard format and it still is more common overall than horizontal writing (横書きyokogaki). However, with the recent increase in the use of computers and the Internet, yokogaki is fast becoming widespread nowadays, so younger generations in particular may feel like yokogaki is easier to read.
Historically, Japanese used to be written only vertically, starting from top to bottom, with the lines proceeding from right to left. The majority of the apparent yokogaki examples back then were in fact some specific cases written with one character each per line by tategaki; that's why these seemingly yokogaki writings are actually read from right to left, unlike modern left-to-right yokogaki. It was only after WWII that yokogaki was officially introduced and became more common. So, it makes sense that my great auntie, who is in her 80s, often says to me, "Oh, nowadays you read and speak yokomoji (literally, "horizontally-written characters", meaning western languages especially English), don't you?". To someone her age, it may feel like the horizontal format characterises western languages and I find it intriguing.
Modern Japanese scripts adopt tategaki and/or yokogaki depending on the type of the writings and their genre. Let's take a look at some examples. Typical ones that employ tategaki include the majority of writing in newspapers (except for some headlines) as well as most books, magazines and manga (Japanese comics). As these are written vertically, they start from what would be the back cover of English books/magazines. Another type of tategaki examples is formal greeting cards and traditional writings. On the other hand, specialists' books in certain genres like music, science, computers, mathematics, languages, etc. are usually written horizontally, as their materials (e.g., musical notation, mathematical formulas, etc.) suit yokogaki better. Also, websites and computer-related writing (e.g., WORD) along with most technological gadgets predominantly adopt yokogaki. Texting from mobiles (携帯 or ケータイ ketai) is also done in yokogaki. There are also many cases which use both tategaki and yokogaki such as product information on their packages, and interestingly, children's picture books (絵本 ehon) can either be tategaki or yokogaki, which means that Japanese children these days are exposed to both formats from early age.
In spite of today's increasing prevalence of yokogaki, it does seem logical that Japanese was originally and still frequently is written vertically, given its orthographic characteristics. Kanji and kana strokes as well as their hitsujun (the stroke order) assume tategaki, and it is even impossible to do some calligraphic styles horizontally. In addition, there are generally many horizontal strokes in Japanese characters. It is said that it's easier for human eyes to recognise the lines in the written words which cross orthogonally with the writing direction; for example, Japanese kanji numerals are 一 (ichi),二 (ni), 三 (san) whereas the equivalent Roman numerals are I, II and III. As pointed out by a well-known Japanese linguist, there are indeed a number of kanji characters which can be differentiated by just one horizontal line: 鳥 (tori; "bird") vs. 烏 (karasu "crow"); 日 (hi; "day") vs. 目 (me; "eye"); 王 (ō; "king") vs. 土 (tsuchi; "earth, soil"); and so on. It seems conceivable that the written characters have been systematically developed in such a way that they can give strong visual impact, in accordance with the vertical/horizontal format for each language.