I think the best thing we can give you on this thread are stories about how we decided to go to med school -- my guess is most people are 'sure' by the time they actually go to med school, i.e., they're decided. I'm not sure if that means "I'm absolutely positive I will want to do this 140 hours per week for the rest of my life and will trade my first born son for the opportunity to do it." It may just mean, I think I have something to contribute here, am willing to put in the time and energy to find out.
My personal story is -- I was dead set against going to med school throughout college. After college I worked in a lab, and shadowed a medical doctor pretty regularly, but thought it was boring as watching grass grow (three hours of rounds, seeing patients who talked incessantly about headaches, etc... it was a neurologist). I tried to get into it, because doctoring seemed like something you should try to get into (probably similar to where you are), but it didn't really click.
Then I shadowed my neurologist into the OR, where they were doing operative monitoring of cardiothoracic surgery... if there was ever a lightning bolt moment, that was it. I just stood there staring over the tent for 8 hours, completely taken. I walked out of that OR thinking, that's what I want to do.
Then I woke up the next morning, and tried to talk myself out of it (I mean, 2 years of getting into med school, 4 years of med school, 7 years of general surgery residency, 2-3 years of CT fellowship). But there was something about the OR -- I just kept going back as often as I could.
Anyway, I'm now a first year in medical school and loving life. I get to scrub in about 1 day a week, follow the patients I scrub for, learn lots of neat stuff in my classes (though some classes are better than others for the future surgeons among us, as you can imagine) and so on.
All of which is to say -- can you learn to love medicine? Yes, of course. Everyone has to learn to love medicine at some point, either before or after med school. But it'll probably be from seeing something you haven't seen before, or doing something you haven't done before, or being around a different group of people. Don't keep hanging out with a rheumatologist if you're bored off your rocker -- try hanging out with the surgeons, or a family practice person, or whatever.
Given the time, energy, and money you're going to invest, I'd recommend having some sense of "I think I could enjoy doing X" before you start it. But, look, there's nothing wrong with having an MD, even if you decide medicine isn't for you in the end. There are worse ways to spend four years of your life.
Best,
Anka