For some people they choose these fields out of a sense of duty to the community but this is rare and not likely to fill the required productive capacity. The end goal should be automating these fields and a communist society run by workers would inherently work towards this goal. However in the mean time incentives like an early retirement and reduced working hours would likely boost numbers significantly. This is a sacrifice though as it means more people are required to do the job and these workers stop contributing to society at an earlier age, depending on the material conditions and specific stage of development this could be much harder to accomplish in which case that sense of duty would have to be reinforced by culture. The socialist transition is no paradise, it requires dangerous work and personal sacrifice to create a better world. There are likely other incentives that could be implemented more easily but these are the first two I thought of.