Albert Barnes on Luke 15:15

Joined himself. Entered the service of that citizen. Hired himself out to him. It would seem that he engaged to do any kind of work, even of the lowest kind.

A citizen. One of the inhabitants of one of the cities or towns of that region, probably a man of property.

Into the fields. Out of the city where the owner lived.

To feed swine. This was a very low employment, and particularly so to a Jew. It was forbidden to the Jews to eat swine, and of course it was unlawful to keep them. To be compelled, therefore, to engage in such an employment was the deepest conceivable degradation. The object of this image, as used by the Saviour in the parable, is to show the loathsome employments and the deep degradation to which sin leads men, and no circumstance could possibly illustrate it in a more striking manner than he has done here. Sin and its results everywhere have the same relation to that which is noble and great, which the feeding of swine had, in the estimation of a Jew, to an honourable and dignified employment.