In the course I am doing (Yan san...) this is taught as having an explanatory function. suki ja nai alone would simply state you don't like something but ja nai n da has a connotation of explaining that you don't like it (hence you may not want to go there?)

母音の無声化（Vowel devoicing）　好きじゃないのだ　　Janaーinoーda f in"o " "o "is　drop become Janainda therefore no meaning ん also 夢があるのではないか　あるの also same kinds phenomenon 　a runo - arun . One of the Japanese Phonetics Phenomenon Vowel + N + Vowel is one of most common case.

じゃないのだ -> じゃないんだ == ja nai NO da -> ja nai N' da