My great grandfather traveled back and forth 4 times. The late 1800s and early 1900s were very tough times in Europe and people came here for work. Some men came alone so they could establish themselves and get a job. If they were planning on staying in the US, they would either send for their family members, or return home and then they would all come back to the US together.
My great grandparents arrived at Castle Garden in 1886 (the port of arrival before Ellis Island was open). They returned to Italy c 1891. My great grandfather came back to the US by himself in 1897. I don't know when he returned to Italy, but in 1901 he and my great grandmother came back to the US (she was pregnant with my grandfather during the voyage). All three returned to Italy c1915, and then my great grandfather came back to the US by himself in 1916 (and I believe he died here that year).
My grandfather married in Italy, and my father was born there. My grandfather came through Ellis Island alone in June 1924; my grandmother and my father followed 5 months later. I don't believe they ever returned to Italy.