We explore the origins of the threshold current temperature dependence in InP quantum-dot (QD) lasers. While the internal optical mode loss does not change with temperature, the peak gain required to overcome the losses becomes more difficult to achieve at elevated temperature due to the thermal spreading of carriers among the available states. In 2-mm-long lasers with uncoated facets, this effect is responsible for 66% of the difference in threshold current density between 300 and 360 K. Spontaneous recombination current only makes up at most 10% of the total recombination current density over this temperature range, but the temperature dependence of the spontaneous recombination in the QD and quantum-well capping layers can be used, assuming only a simple proportional nonradiative recombination process, to explain the temperature dependence of the threshold current density.