Whenever you express a doubt an uncertainty, an opinion, a wish, the sentence that is
the direct object of the primary verb (pensare, sperare, credere, augurarsi, etc.) ahould be in the mood 'congiuntivo'.
The rule is simple if you translate into English: SUBJUNCTIVE.
That reminds you that you are expressing the point of you of the SUBJECT (the person).
Different thing would be if the verb is related to 'sapere' (to know).
Here you would use in the subordinate the verb that points (= indicatre) to the reality, i.e. the 'indicativo' mood.
Example: Io spero che lei torni, ma so che non tornera`.