The word "normal" will have many meanings, depending on the context. However, I think that in general "being normal" or "behaving normally" is a good thing. Being normal doesn't mean that you are the same as everyone else. Actually it's normal that there are differences between people, that we have different interests, views of the world, ways of life. Therefore I disagree with people saying that "normal" equals conformist, boring, etc. Actually "normal" people are supposed to be free-thinking, possibly creative people, people who care to make the world a better place. Normal people might be a minority. Of course at some stages of history there can be societies where abnormal behaviour becomes the rule. For example in Nazi-Germany in the views of the regime people supporting the regime were often hard-working, decent people, who apparantly did not do anything wrong. Even though at some point they were a majority, it was their blindness for the complete abnormality of the political situation and racism in their society which was abnormal. The same can probably be applied to the current indifference of the majority of nations towards the destruction of our natural environment. Still it's a tricky debate. At some point in history, things like cannibalism or human sacrifices must have been considered as "normal", according to archaeology. We who have our 21st century point of view can never consider that as normal.
What is classified as "abnormal" behaviour is generally behaviour which is self-defeating, behaviour which is damaging to yourself, other people or the environment. It can be caused be "primitive" impulses, which at some point in human evolution perhaps allowed us to survive, but which now have become useless or damaging. Examples of personal behaviour would be violence, bullying or criminal behaviour towards others. Abnormal would also be extreme neglect of your own "duties", which will possibly lead to great problems for yourself or others at some point in the future: ignoring people who are in urgent need of help, even though you are perfectly able to help them. Neglecting your own health, for example, possibly causing great trouble for yourself or people close to you later in life. Badly managing finances.
Social phobia is considered abnormal because it will seriously impact many aspects of your life and limit your possibilities in life, and prevent you from deriving happiness from interacting with others, and it can be the root cause of some of the above mentioned other abnormal behaviours.
