For Christopher Boone, a loving family environment is one of the most important things in the world. While this type of environment is important for anybody, it is particularly important for Christopher. This is due to the fact that, although the book never mentions the words "Autism" or "Asperger's Syndrome," it is obvious that Christopher has a high-functioning form of one of these conditions.
For somebody on the spectrum, unconditional love, stability, and a safe environment are essential. He has a number of quirks which make him unusual in societal terms, and it is of the utmost importance that he is surrounded by people who understand him or who are at least willing to attempt to understand him.
To list a few examples of Christopher's quirks, he cannot be touched, he finds fiction incredibly difficult to understand, he will not eat anything that is brown or yellow in color, and if he sees three red cars in a row on his way to school, he thinks it means that he will have a good day.
Christopher needs to be surrounded by people who understand that when he finds a situation difficult, he is not simply being disobedient or obstinate.
After Christopher's arrest, he and his father share an "I love you" gesture that the family has invented. It is clear that it is his father's loving and tolerant stance in raising Christopher that has made life bearable thus far.