Farid, the guide whom Amir hires to take him around Afghanistan and who incidentally acts as something of a protector, can't really understand why Amir has even returned to Afghanistan, and he finds it even more difficult to understand why Amir deliberately chooses to stay in Afghanistan for some boy that he has never met. Note what he says to Amir when he takes him to his father's old home, where Hassan and his wife were killed:
How much more do you need to see? Let me save you the trouble: Nothing that you remember has survived. Best to forget.
Farid therefore counsels Amir to move on and not to live in the past, and his position about Sohrab is exactly the same. Note what he asks Amir:
What I mean to ask is why that boy? You come all the way from America for... a Shi'a?
From his perspective, Amir's actions are foolhardy in the extreme. Why would he come back and try and rescue a boy that is a stranger--at great personal risk to himself--when he has such a comfortable existence? Note, too, that Sohrab's ethnicity and religion are also factors that cause Farid to discriminate against him: just a Hassan encountered racism and discrimination, so his son does too.