Why can’t Siddhartha learn to love another? What prevents him from embracing that feeling throughout the rest of his journey?

He just can't yet

Explanation:

Young Siddhartha, full of grief, attends his mother’s funeral. He lives in the ferryman’s hut but will not eat. Siddhartha respects his grieving. He finds that the boy is spoiled, and used to rich things and the attention of his mother. The boy does not know Siddhartha and will find it hard to cope with such minimal conditions, so Siddhartha is patient and cares for him. But though at first, Siddhartha felt so enriched by the thought of knowing his son, as time goes on, the boy’s mischievous ways and surly attitude toward Siddhartha drains him of that initial joy. But still Siddhartha loves him and suffers through it. He and Vasudeva divide the work between the river and the hut so that Siddhartha can stay with his son.

Siddhartha has faced challenges throughout his life, but the arrival of his son and his son’s grief overtakes his mind and heart like nothing else so far. The familial bond seems to have an effect that no other bond has, not even the bond he had with Govinda or with his revered elders like Vasudeva and Gautama. It is the paternal bond that makes Siddhartha excuse the boy’s worldly sullen attitude that, in contrast, Siddhartha had greeted with such scorn when he first encountered it in the town.