The emergence of new passive constructions in vulgar Latin and early Romance
If you look over the pages on the passive voice in Classical Latin, you will gather that participles (AMATUS) are essentially past in aspect. So, AMOR (E:I am loved; SP: soy amado) is present in aspect, where AMATUS SUM (E:I was loved, have been loved; SP: fui amado, he sido amado). The result is the same: I am loved, am 'having been' loved, was loved, have been loved - the loving, however may or may not be still happening. If we look at PORTA APERITUR and PORTA APERTA EST, past action has clearly generated a present state; this is expressed using the verb estar in modern Spanish. Furthermore, adjectives and past participles are identical in aspect and position when used with the verb to be, whether this be ser or estar in modern Spanish. The cumulative effect of all of this, then, was a new generation of passives, supplanting verb forms such as AMOR above.
In vulgar Latin (sermo vulgaris), the active voice was preferred in a number of impersonal passive constructions, such as HABET (SP: se tiene for one has and hay, for there is or there are); DICIT > se dice (impersonal passive), just as POTEST > se puede and DEBET > se debe. This did not occur in Classical Latin (sermo urbanus). Sometimes the third person plural was used instead (eg DICUNT - it is said that, NARRANT - it is related that, SCRIBUNT - it is written that).