Med students know very little about what a specialty is really like, which is fine, because you simply can't without practicing it for a few years. So the first division is: do you love to cut and be in the OR or not? That's the first branch in the decision tree. After that, it's all about the Stars Theory. What is the Stars Theory you might ask? My theory is that a med student picks a specialty for which all the right 'stars line up'. What the heck does that mean? "When the stars align" means things fell into place personally for you lifting one specialty above all the others. The rotation went well for you, the department director seems to click with you more than others, the subject matter or anatomical focus fascinated you, you may have scored better on the shelf, a patient in that field resonated with you, an attending said one thing to you that struck a chord, you were in the mood for doing X and Y when on that day X and Y happened, someone gave you a compliment that was pivotal, etc. It's extremely subjective. Almost like deciding whom to marry. The stars line up between the things you think represent that specialty and you (beware that your concept of self will change as you reach certain milestones in life).
Now that I'm practicing for a year after fellowship, I can tell you what I truly appreciate about Psychiatry. The mind is the final frontier, and I like psychosis, depression, mania, and those things. Psychiatry is an amazing balance of close patient interaction, fascinating work that changes daily, presentations that are not clear cut (I love that), pretty high pay in the right setup (and its only going up), very high demand everywhere, can work as little or as much as you want, low stress most (but not all) of the time, and patients who have dramatic improvement where about once a week you get a hug. It's also the one specialty where how you speak to a patient, the interpersonal dynamic, is part of the treatment. If you're not good at that, then you should really stay clear.