I worked in an ER as a PCA while still pursuing undergraduate studies. This solidified my interest in medicine, and I thought then that I wanted to be an EP. I tried to keep an open mind in med school, and even considered other specialties. But I kept coming back to EM.
I hate rounding, I hate call. I like seeing diverse pathology and seeing people of all ages. I hate to see the same patient day after day in the hospital. I like the pace of the ED, and the fact that when you go home, you don't have to be bothered all night and carry a pager.
In your 3rd year clerkships, you will soon realize what you like and don't like. I think that that is when most people know what they want to be when they grow up.