The better we know people, the more likely we are to find fault with them.
familiarity breeds contempt
Long experience of someone or something can make one so aware of the faults as to be scornful. For example, Ten years at the same job and now he hates itfamiliarity breeds contempt. The idea is much older, but the first recorded use of this expression was in Chaucer's Tale of Melibee (c. 1386).