This follows from how reflexive pronouns are used when a verb is conjugated in German. Here's one verb with accusative and one with dative reflexive pronoun:
Ich freue mich. Ich mache es mir bequem.
Du freust dich. Du machst es dir bequem.
Er/sie/es freut sich. Er/sie/es macht es sich bequem.
Wir freuen uns. Wir machen es uns bequem.
Ihr freut euch. Ihr macht es euch bequem.
Sie freuen sich. Sie machen es sich bequem.
So in third person singular and plural, the reflexive pronoun to use is always "sich", while in first and second person, other than in English, the "normal" accusative or dative pronouns are used (i.e. the ones that would also be used if the subject was a different person). This is possible because they are unambiguous in first and second person but not in third person. E.g., in third person plural, "Sie machen es ihnen bequem" (They make them comfortable) and "Sie machen es sich bequem" (They make themselves comfortable) are two different things.
The formal "Sie" address is grammatically the same as the third person plural, and that's why it uses sich:
Sie lassen sich Zeit. (They are taking their time or You are taking your time)
Or in imperative:
Lassen Sie sich Zeit!
Whereas in second person singular, as seen in the conjugation above, the "normal" pronoun dir is used:
Lass (du) dir Zeit!