Good question.

All life that we know about uses carbon. Carbon is very important because it can form large numbers of strong chemical bonds with other atoms. This enables carbon to form the backbone of large, complicated molecules, and life uses complex molecules to store its information. Other elements with similar chemical properties (such as silicon) form large numbers of bonds, while other light elements (such as oxygen) form strong bonds but in fewer numbers.

At some level, though, carbon is necessary for life because we observe that all life uses molecules made out of carbon. Are there any other ways to make life that don't rely on carbon? We've never seen any, but that doesn't mean that they don't exist.