1. People can take advantage of you. Some people confuse niceness for weakness. And if they think you won't stand up for yourself, they'll try to manipulate you.
2. They might question your motives. Are you REALLY that nice? Or are you just trying to manipulate THEM? You also might seem like a brownnoser to your boss.
3. You can seem boring. Nice is great as long as you still have some personality. But if "nice" is your defining characteristic, you'll come across as passive and bland.
4. You might start sacrificing too much of your time by helping. If you're constantly helping coworkers, you'll fall behind on your own work. Or even if you don't, you end up giving them a lot of your energy.
5. People might stop listening to you. Being too nice can make you seem like a pushover. So just make sure people realize you can say no and put your foot down if you need to. Especially if you're anyone's boss.
6. If you suddenly change your tone, you'll seem like a jerk. It's not fair. But if you're always nice before finally standing up for yourself, the contrast can catch people off guard. So they'll hold it against you more than someone who's NOT always nice.