Possible resons for rejection of your application are given below-
1) If you have applied with such an email account that is already in use in google adsense they will reject your application
2) If you use such an URL that is not owned by you then they will reject your application
3) If you transgress any Policies of adsense they may reject your application
4) If you have not a good design of your website/blog,most probably is also the reason for rejection
5) If your website is in baby stage they will not approve your application,make sure that your website is more than 6 month of age before applying
6) Full of copy paste(PlGIARISM) contents is also the reason for disapproval.
Peace be upon you