Why does the temperature of a pro star increase over time?

The temperature of a protostar increases over time due to gravitational contraction. As the protostar gathers mass from its surrounding dust and gas, gravitational forces compress the material, raising its temperature. This process continues until the core reaches sufficiently high temperatures for nuclear fusion to ignite, transitioning the protostar into a main-sequence star. Additionally, energy from the infall of material and the release of potential energy during contraction contribute to this increase in temperature.