I think many interns switch specialties during their intern year prematurely. Internship gives you no great insight into your choice of specialty and career options whatsoever. Internship sucks for everyone, regardless of the specialty, yet most interns think this is a factor that correlates directly to their choice in specialties and don't realize that it's a ubiquitous feeling and universal aspect of residency in general. Quitting anything during your first year is always a bad idea in my opinion. Let's say you feel the same way by year 2 or 3, well... by then it almost makes more sense to finish your training and then pursue something else if you so choose.
All that being said, I think everyone becomes a bit burnt out and jaded during residency and has 2nd thought sometimes but I think that largely is due to the workload. Once you're out and have more free time, you're likely to regain your passion, enjoyment and interest. I'm in EM and after a really arduous string of shifts or month, I feel jaded and cynical. Give me 3 days off and I'm itching to get back into the ED and am completely revitalized. Being overworked, which just goes along with residency, can drastically change the way you feel about being at work, regardless of anything. My 2 cents.