All Italy joining the central Powers would have done is give the Franco-British fleet something to actually attack. Italy is extremely vulnerable to an enemy who can gain naval superiority, as she found out in WWII, and a joint Italian-Austrian naval axis would be no match for the Franco-British one. With Germany mostly hiding its surface fleet from Britain, Franco-British assets could have easily been transferred to the mediterranean, perhaps overcoming an Austro-Italian flotilla trying to check access through the Strait of Gibraltar, and have no shortage of targets to easily bombard and docked fleets to destroy piecemeal.

One of the only things stopping that from happening in WWII is that the Regio Aeronautica was actually pretty good early in the war. Easily Italy's best fighting branch with good planes and great pilots in reasonable numbers that could stand up to the British on their own at first. That combined with the neutralization of the French fleet, delayed the Franco-British ability to pummel Italia for a few years.

Italy would have had no similar protection, however, in 1915, and nearly the whole coastline was vulnerable to coastal raiding,bombardment, and eventually invasion, as the US-British naval coalition eventually proved. Minus the US but plus France, which at the time was a major naval player in its own right, Britain would have easily achieved the same success even with the small, technologically backward Austro-Hungarian fleet muddying the waters a bit.

So with that being siad it was impossible to imagine aligning with the Central Powers ending well for Italy. They simply had no ability to properly aid them in the one theater that mattered -- the Mediterranean naval theater. Thus Italy had a choice between staying out of the war or picking the side that was astronomically more likely to rule the Mediterranean during and after the war, which was the Entente.

Sacroegoismo was simply a reasonable argument that if they sided with the Entente they might get some desired territory out of it, and that self interest aligned them with the security of the seas offered by the Entente, which was essential to Italian survival. Since they only had 2 options they chose the one that gave them the best chance to gain. Simple enough.