Asset(A) = Equity(E) + Liability (L)
A= E + L ... At year zero
After end of year one, let say you get profit and all profit retained in a business, automatically increases equity
A= E + L + Profit(P)
But Profit (P) = Income(I) - Expenses(Ex)
Therefore
A= E + L + I - Ex
A + Ex = E + L + I
Therefore Asset and expenses are debit in nature and Equity,liabilities and Income are credit in nature