The Liquidity Preference Theory suggests that investors demand higher interest rates for longer term investments because cash is considered the most liquid asset. There are three motives for demanding money: transaction motives for daily needs, precautionary motives for unexpected events, and speculative motives based on interest rate fluctuations. The total demand for money is the sum of these three motives and is determined by income and interest rates. Interest rates are set by the point where the total demand for money equals the fixed money supply as determined by the central bank, with the demand curve sloping downward and the supply curve being vertical.