For many years, the operation of long-wavelength (1.3 micrometer and 1.55 micrometer) vertical-cavity surface- emitting lasers (LW-VCSELs) was restricted to low temperatures. Continuous-wave lasing above room-temperature (up to 64 degrees Celsius) has been achieved only recently. The strong temperature sensitivity of the lasing threshold is well known from their edge-emitting counterparts, but LW- VCSELs exhibit principal differences. Focusing on the so far most successful concept of wafer-fused LW-VCSELs, the physical mechanisms are analyzed that affect their temperature sensitivity. The analysis includes optical gain, carrier losses, optical losses, and self-heating. Photon absorption within the valence bands and auger recombination are found to limit high-temperature lasing. Scaling down the active area by lateral oxidation, VCSEL self-heating can be reduced despite a rising thermal resistance. Based on an excellent agreement with measurements at lower temperatures, numerical VCSEL simulation is employed to investigate laser operation up to 120 degrees Celsius. With minimization of threshold current and absorption losses and with proper adjustment of the gain peak wavelength, high-temperature continuous-wave lasing is predicted that is less temperature sensitive than in edge-emitting lasers.