Why are cats attracted to catnip?
Nepeta cataria(Catnip, Catmint, Catnep) is a plant belongs to the family Lamiaceae.It is a  herb that has been recognized for its medicinal properties for many centuries. Over time it has been used in remedies to cure internal cancer, smallpox and scarlet fever.Also it was believed that a dense growth bordering fields helped to protect crops because of its ability to repel rats.

Major Active Constituents of catnip includes a and ß nepetalactone  citronella , a and ß citral , limonene , geraniol , thymol , and carvacrol  epideoxyloganic acid and 7-deoxyloganic acid


  Nepeta cataria has a carminative, anti-spasmodic, astringent and anti-flatulent effect on the gastrointestinal system, particularly on the stomach . It is also a diaphoretic and antipyretic in times of fever . Nepeta cataria is similarly recognized as a tonic, mild nervine, and sedative, with a slightly emmenagogue action . With respects to cats, Nepeta cataria produces an aphrodisiac effect. 


http://scu.edu.au/scps/index.php/125/

A research paper published in 1994 states that, 


  Catnip was  the most promising candidate lures tested. The 'catnip-response' is 
  inherited as an autosomal dominant (Todd 1962). In trials, behavioural observations showed 
  that 50% of the subjects (including all four domestic cats and 8 of 20 feral cats) showed the full catnip response (B. K. Clapperton, unpublished data). This includes sniffing, licking, rubbing and head-over rolling (Tucker and Tucker 1988). Nine of the remaining 12 feral cats approached and sniffed at the catnip samples. This suggests that, although not all cats are genetically determined to respond to catnip , these odours may act as more general attractants.


http://www.southwestnrm.org.au/sites/default/files/uploads/ihub/clapperton-bk-et-al-1994-development-and-testing-attractants-feral-cats-felis-catus.pdf 


  When cats inhales nepetalactone it stimulates the olfactory bulb, the part of the brain that processes odors. The olfactory bulb then interacts with the amygdala, the brain region associated with emotion and decision-making, and hypothalamus, which controls a variety of bodily functions. From the hypothalamus, nepetalactone stimulates a sexual response in cats that are genetically predisposed to sensitivity to catnip. (About 20 to 30 percent of cats don't seem to react to the plant.)this is why mostly adult cats shows significant addiction to catnip.


http://www.compoundchem.com/2014/06/08/catnip/


  The catnip stimulates an Innate Releasing Mechanism(IRM) in domestic cats that elicits a predictably playful behaviour pattern apparently independent of experience and learning(Ewer,1973 and Leyhausen,1975) ,trans-nepetalactone is reported to produce behavioral respondse in cats.It has been verified by Todd in 1963 that capnip is mediated by Olfactory and not gustatory stimuli.Catnip sensitivity has been related to a dominant autosomal geneand to the estrous cycle by Todd.The reports shows catnip elicits behavioral respondses in domestic and wild cats, but these responses are not shown by female sex.In addition, it has been suggested that catnip mimics a pheromone found in cat urine, however evidence supporting this theory is scant.(Todd 1962)


http://link.springer.com/article/10.1007%2FBF00987747#page-1