It's not entirely clear if Justin Bieber (I think he's from Toy Story 3) solicited Usher's advice or not, but Usher's offering it anyway:
“I tell Justin you’ve got to love all your fans without falling in love with them. I can’t say I’ve never dated a fan but it’s not a good idea. I tell him to understand that you are a fantasy to them and you should be mindful of that,” Usher said.It's bloody good advice.
“And I encourage him to not take success so seriously so early. Have fun with it. And make friends!”
For Usher, he shouldn't date his fans because they like songs by Usher, and who wants to hang out with someone like that?
For Justin Bieber, he shouldn't date his fans because they're eight years old and he'll end up on some sort of register.