Here's why:
* Metaphor: A metaphor is a figure of speech that compares two unlike things directly, without using words like "like" or "as".
* In this case: The furrows (lines in the soil) are being compared to the act of bearing or giving birth. The speaker is suggesting that the hard work and dedication of their father in the fields (represented by the furrows) are the reason for their existence.
The comparison is not literal, but rather figurative. The furrows didn't literally bear a child, but the metaphor creates a powerful image of the link between the speaker's father's labor and the speaker's own life.