Classical Greek still had "medium voice" between "active voice" and "passive voice" as a separate verb form. This voice is used for reflexive use and verbs expressing emotions.
This voice, probably once part of a common language ancestor, is only found in traces in other languages and in German often is considered to be a complete different verb. Latin has inherited "deponentia", verbs with passive forms (a fair number of medium voice forms are identical to passive forms even in Greek) and active (usually intransitive) meaning.
For German, the difference is often that the reflexive/self-referential use of a verb variant is declined strongly (changing its vowels and adding "-en" to the past participle) while the transitive variant is declined weakly (retaining its vowels and adding "-t" to the past participle).
So we have: "ich erschrecke dich", "ich erschrecke [mich]" ("mich" optional), but "ich erschreckte dich", "ich erschrak" ("mich" impossible), "ich habe dich erschreckt", "ich habe mich erschrocken" or "ich bin erschrocken" ("mich" required in the first, impossible in the second where arguably "erschrocken" is not as much a verb form but an adjective in usage).
Similar "ich lege ein Puzzle", "ich liege", "ich legte ein Puzzle", "ich lag", "ich habe ein Puzzle gelegt", "ich habe gelegen" where the medium voice derived form is considered a separate verb in current grammar.
Also "ich setze dich auf einen Stuhl", "ich sitze auf einem Stuhl", "ich setzte dich auf einen Stuhl", "ich saß auf einem Stuhl", "ich habe dich auf einen Stuhl gesetzt", "ich habe auf einem Stuhl gesessen": again the depth current grammars descend to is that we are talking about two different unrelated verbs with different forms that magically share the same root.
Of course, like with most things in language, this rule has exceptions: "helfen" is conjugated strongly although being a transitive verb. But when one verb (or one "apparent" verb) has both a strong and a weak form of conjugation, chances are that the meanings are related and that the strong form of conjugation is used for an intransitive or reflexive meaning while the weak form is for transitive use.