Immigration should have as its goal integration, not disintegration or segregation. The immigrant should not only desire to assume the benefits but the responsibilities of joining into the full fellowship of the nation. By becoming a citizen, a person becomes part of a broad family over the long term and not a shareholder in a joint stock company seeking only short-term self-interest.
Secondly, Saint Thomas teaches that immigration must have in mind the common good; it cannot destroy or overwhelm a nation.
This explains why so many Americans experience uneasiness caused by massive and disproportional immigration. Such policy artificially introduces a situation that destroys common points of unity and overwhelms the ability of a society to absorb new elements organically into a unified culture. The common good is no longer considered.