@Akashi-san: It's more likely that nobody ever told Aishira 'no' or admonish her much. And since she's never been told about not getting close to a man who has fiancee (we haven't seen her with anyone else besides Aqua), especially since she likely could do this before Aquasteed got Tiara, she doesn't realize it's no longer acceptable.
After all, given precedence of what kind of catastrophe could come from making the favored person unhappy, nobody is willing to risk that.
That said, since it depends on Aishira's mood, as long as Aishira herself doesn't mind Tiara marrying Aquasteed, there won't be any problem with the sea faries.
Though I'm not sure how many people are aware of Tiara's blessing by the forest faeries.
@sweet-potato: Because clearly letting her stress herself to death SURELY is a better alternative right?
Like how a doctor should just let someone who drink bleach just die because clearly they chose to drink bleach, so whatever happen to them must be what they want.
He is making this intervention BECAUSE Tiara is in such bad shape from her nightmares, he even noted how the sleeping spell worked so fast because of how tired she was.
Him refusing to return her to Aquasteed is probably a mix of testing/venting anger at him for leaving Tiara alone that she's in this state, which honestly you can argue is him even holding back considering the kind of devastation recorded in the past (and that's the sea faeries, the friendliest of the bunch)
@emMeBi86: My guess is because Keith actually take actions to care for her.
Yeah he kidnapped her and put her to sleep, but it's because Tiara's condition was so bad that he decided to make the intervention.