I noticed that all Italian verbs has the same conjugation for indicative present and subjunctive present for subject noi. The form also seems to have a closer relationship with the subjunctive forms (e.g. abbiamo is closer to abbia rather than ha, so is siamo to sia). I can't even find a single exception (is that right?). What is the story behind this "incidence"?
According to A Linguistic History of Italian by Mardin Maiden,
The reasons for the spread of the inflection -iamo are obscure, although it clearly originates in the Latin second and fourth conjugation subjunctives in -eamus and -iamus.
Following the text, here are the observations:
The Latin active present subjunctive appears to be the origin of all these modern -iamo. The ending -iamo (together with 2nd-plural -iate) was originally restricted to a small set of frequently used verbs, such as HABEAMUS > *a'bjamo > abbiamo and SAPIAMUS > *sa'pjamo > sappiamo.
Based on those verbs, the endings -iamo and -iate later found their way to the replacement of the subjunctive of all conjugations, replacing their "original" forms (e.g. cantiamo = cant- + -iamo, instead of *cantemo < Latin CANTEMUS). By mid-13th century, the replacement had already finished. The ending -iamo also had a consistent appearance in indicatives of the first conjugation (e.g. portiamo), but still within a limited range (e.g. both vendemo and vendiamo were found, but only dormimo, no dormiamo). (Wanner, 1975)
Having established a bridgehead in the first conjugation, the ending -iamo then spread to the first person plural indicative of remaining conjugations. Following the prevalence of -iamo in indicative, one of the basic words *semo (< Latin SUMUS) also got replaced by siamo, favoring the ultimate generalization in --iamo endings in all verbs.
However, as stated by Martin, it's still unknown why the 2nd person plural indicative didn't get replaced by the subjunctive along with the move (e.g. we have indicative voi cantate but subjunctive che voi cantiate).