Long-term interest rates are considered to be determined mainly by two factors: first, the forecast of future short-term interest rates, and second, an extra interest rate required for risk associated with long-term bond investment.
Regarding the first factor, if the future economic growth rate or the future inflation rate is forecasted to increase, for example, then the future path of short-term interest rates will rise accordingly and one can expect a higher return by investing repeatedly in short-term bonds.