Why are worldly relationships considered undependable for spiritual attainment when compared to the Guru-disciple connection?

According to the teachings in Chapter 45, worldly relationships are not reliable for the spiritual goal because they are rooted in transient attachments and self-interest. The text posits that people who won't even help in worldly life cannot be expected to help in spiritual matters. For instance, a mother's affection is shown, a son eyes the wealth he can claim, and a wife mourns her own potential widowhood. None are true companions in the spiritual goal. In contrast, the Guru is described as the 'true Brahman,' the eternal reality. The bond with the Guru is focused on renouncing the transient and meditating on the eternal, which is the very means to achieve liberation.