Why asset must equal to liabilities in the balance sheet ?

Company's assets must be equal to (Liabilities + Equity). That is:
Assets = Liabilities + Equity.
It reflects how the company's assets are financed. Company owns assets may be due to the equity from shareholders or from borrowing.
Your question is wrong.
Your balance sheet with liabilities greater than assets reflects that you have net deficit on financial position. Technically, you are insolvent.

首先更正, 係 balance sheet 上面, assets 並唔係等如 liabilities, 而應該係:

Assets + Liabilities = Capital

呢樣係會計上成文既準則.

至於你個人既 balance sheet 如果係 liabilities 大過 assets, 同以上準則亦冇衝突, 因為你既個人 formula 就係:

Assets + Liabilities = Deficit in capital

即係負資產!

所以　Assets + Liabilities = Capital　仍然係成立既！