A balance sheet should always balance. The name "balance sheet" is based on the fact that assets will equal liabilities and shareholders' equity every time.

Why both sides of balance sheet are equal?

It should always balance because every individual transaction impacts both sides. Where the money came from and what it's being used for. So, if the double-entry accounting process has been followed correctly, it'll always be the same.

How do you know if a balance sheet is correct?

with assets listed on the left side and liabilities and equity detailed on the right. Consistent with the equation, the total dollar amount is always the same for each side. In other words, the left and right sides of a balance sheet are always in balance.