Hello. It's because "divertirsi" is a reflexive verb and it is different from "divertire", that is non - reflexive. Dirvertirsi = the action has effect on the person that is speaking (es: NOI ci divertiamo) Divertire = the action expressed by the verb has effects on a third person (il clown divertiva il pubblico) Reflexive verbs use "essere" as auxiliar, non reflexive "avere" So: "Noi ci siamo divertiti" "Il clown aveva divertito il pubblico". I hope I solved your doubt.

The passato prossimo of reflexive verbs is always formed with "essere" as the auxiliary verb. So it's "mi sono divertito/a", "ti sei divertito/a" etc. even though non-reflexive "divertire" has "avere" as the auxiliary.

Grazie mille!