I guess the study life balance. You might not get to visit or attend every event that you want to. It sometimes can get isolating. For example, I have family in another country. My only close relative, who I consider a brother, was getting married. I asked my school in advance (at the beginning of the academic year~ 6/7 months in advance) if I could take off for one week and they made it seem like it was not possible. Looking back, I wish I fought for it more because it could've been possible if my professors were a little lenient. But even to this day, it's one of the things I regret the most. If it's important to you, fight for it. But, yes, sadly, there will be things that you have to miss: birthdays, holidays, etc.
Also, medical school is a completely different ball game. It's different from college; you can't cram the night before and think you will pass. You have to have the endurance and patience to study every single day. It's good to make a good group of friends so that you can support each other throughout.