Speech 讠 tied to the self 己: taking words into yourself is remembering; setting words down is recording. Memory and writing share one character because writing IS external memory.
记得 to remember, 忘记 to forget, 日记 diary ('daily record'), 记者 journalist ('one who records'). Every studying session ends with 记住了吗? — 'has it stuck?'
WORDS attached to YOURSELF — RECORDED, REMEMBERED.
忘记 'forget' pairs the two opposites: 忘 losing from the heart, 记 fixing with words — forgetting as failed recording.