First of all, in order for an abnormal behaviour to be diagnosed as a psychological disorder its symptoms have to be persistent and, secondly, they have to impede everyday personal functioning. If an abnormal behaviour doesn't constrain or impair someone's life, then it's not a psychological disorder. For example, if someone is obsessed with washing his hands and he wants to wash them every once in a while because he thinks they're dirty, this isn't a psychological disorder as long as it doesn't influence his personal life in a negative way. If, on the other hand, this habit makes the specific person isolate himself from social contact because he thinks that people are going to make his hands dirty, then this is a behaviour which constrains him in a way, changing his personal life, and it could be considered a psychological disorder.
"in order to determine if abnormal behavior can be considered a psychological disorder, it must be determined that the behavior is persistent, goes against the rules of society, causes significant discomfort, is maladaptive, or poses a danger to the individual or others." -edg
