There is no clear cut answer on whether or not a 2nd time applicant has a tougher time getting in or not. Every 2nd-time applicant vary on what they were lacked in during their first time applying. Lets take two made up unsuccessful applicants here applying to the same school as our example:
Applicant X:
GPA: 3.34 overall, 3.42 Science
DAT: 17 AA 16 PAT
Rec letters: Poor
Extra: none
Orientation to dentistry: none
Applicant Y:
GPA: 2.97 overall, 2.89 Science
DAT: 18 AA 18 PAT
Rec letters: Great
Extra: tons (too much socially)
Orientation to dentistry: dad's a dentist
For Applicant X, (s)he'll need to retake the DAT and shadow a dentist to increase its potential to become a competitive applicant while Applicant Y only needs to increase its GPA to become competitive. The point here is, everyone's situation is different. Some dental school WILL share with you what you need to do to better your chances next application cycle, while other dental school won't tell you ****!
In my opinion, if you get denied in your 1st attempt, then there is a reason or two that you were denied. Those red flags (whatever it is) will be noted on your application file in dental schools that you've applied to. It is more challenging for second time applicants because:
1) red flags noted in your file
2) there are many more fresh applicants each year applying that have better number than the 2nd-time applicants
3) some dental schools will look at new applicant's file first before a 2nd-time applicant's file even both application arrived at the same time
4) 2nd-time applicant have to demonstrate THAT much more to show you're qualified (adcoms are more harsh on your application or expect more)
5) it places more emotional stress on the 2nd-time applicants themselves
Advantages of being a 2nd-time applicant (looking on the brighter side):
1) the school knows you (or at least they should, if they don't then maybe that is one of your "red flags")
2) demonstrate your motivation and determination for the profession and/or that particular dental school
3) know the application process (know what schools to apply and what schools to not apply to)
4) chances to experience a Post-bac program or earning a graduate degree (MS, MA, MBS, etc)
There is no clear cut answer on "why do 2nd-time applicants have a tougher time getting in?", there's just simply much more a 2nd-time applicant needs to do than a first time applicant need to do!
<--- what is this?