Italy switched from the Triple Alliance to the Entente, (Although Italy didn't do much with the Triple Alliance, AND they did not declare war on Germany) this was the case as the Entente promised Italy land and glory.
This hurt Austria-Hungary (who they declared war on) and the Triple Alliance as Italy was very strong at the time (and was considered to still be in the Alliance).
Germany also felt betrayal as the members of the Triple Alliance promised to defend each other during war.
If you need help with an explanation, feel free to ask!