Why did the seafarer keep returning to the sea?
Even though the seafarer spends the first part of the poem explaining how awful, lonely, and cold sea life is, he then says that he keeps going back to it voluntarily.
The time for journeys would come and my soul
Called me eagerly out, sent me over
The horizon, seeking foreigner's homes.
His "soul" calls him out. This is like saying he calls himself out or there is something in his soul that calls him to the sea.
He adds that he still has fear for what Fate has in store for him. Still, he chooses to go. In subsequent lines, he says that all the comforts of life on land "admonish that willing mind / Leaping to journeys, always set / In thoughts traveling on a quickening tide." The comforts of land urge or warn him (admonish) not to go, but it is his own willing mind that chooses. His "soul roams with the sea." The open ocean excites him. There is something incredibly alluring about the treacherous journey of the sea. The cold loneliness and danger along with the excitement of a journey are two sides of the same coin. The excitement he gets from a life at sea must be great enough to overcome the loneliness and isolation he experiences. The life at sea might also be more meaningful to him because of the struggle. Such a notion lends itself to the idea of life as a struggle and a journey.
In the second and third parts of the poem, the speaker shifts to notions of life as a journey. He will stress the importance of God's will (Fate) and how earthly pleasures (such as those he describes on land) are meaningless in the end and something souls (in Heaven) have no use for.