Would you rather they lie to you? They are trying to provide you with solid advice- medicine can be a pretty terrible field for anyone without a masochistic streak. They aren't kidding, they are telling you straight up they would not do it again. I'd say half the residents and attendings I've met strongly feel medicine is not a very good profession to be in, and would not do it over again if they had the chance.
I don't even know how I feel about it some days, but most days I'm fine with it. It certainly isn't puppies and rainbows, and many days are pretty damn miserable, but most jobs are. I guess the real thing I'd say to most premeds is, "Look, medicine is just a job, and it's a job you burn most of your youth getting ready to do, only to find it's not all that much different than any other job in the end. It's not all bad, it's not all good, it just is. So if you think you've got some fluffy joy-filled dream world where you're some hero that's saving lives left and right, I'd say you should re-evaluate your priorities, because most likely you'll be doing a lot of paperwork, a lot of dealing with bureaucrats, a little bit of direct patient care, a lot of second-guessing yourself, and a little bit of feeling like you actually made a difference every once in a while.
You're going to kill people, and a lot of the time, you won't know it, and even when you do know it, it'll be something that you couldn't have expected and couldn't have known. You're going to give people some of the worst news of their life, and they're going to remember those words for the rest of their days. And a lot of the time you'll just be spinning your wheels, or helpless in the face of death and disease, or crippled by rules and red tape and endless preauthorizations and subsequent denials. At the end of the day, it's a dirty, tedious, emotionally draining, physically exhausting, difficult, stressful job where the pain flows broad and deep while moments of satisfaction can be few and far between. So hey, you think that's all good, welcome aboard kiddo. If not, I'd seriously rethink your priorities."