理由
りゆう · riyuu · JLPT N4 · noun
reason
理由 = logic (理) + origin (由): the logic something comes from
理 is the logic of 理解 (understand) and 無理 (unreasonable); 由 means source or cause, as in 自由 (freedom, from oneself). A reason is the logic an action springs from. Sound hook: ri-yuu, as in the reason is YOU.
理 — jade radical 玉/王 + village 里 (phonetic)
理 is a semantic-phonetic compound: the jade radical 王 (玉) on the left signals meaning, while 里 on the right gives the sound. It originally meant the natural veins or grain in a piece of jade, then extended to 'pattern,' 'order,' 'principle,' and finally the modern senses 'reason' and 'to manage.'
由 — pictograph of a gourd or vessel with a stem
由 originated as a pictograph showing a gourd, fruit, or vessel with a sprout or stem emerging from the top. Over time the meaning shifted abstractly to 'come forth from' or 'origin', giving modern senses like 'reason', 'cause', and 'from'. It is unrelated to 田 despite the visual similarity.