Never seen Kuuga, so I'm gonna have to take your word for that.
I don't think Kouta's power-ups come during times when he was in a negative direction though. Rather, I think it comes in times where he had a choice of what to do and couldn't find the motivation to push himself to make that choice, thus needing someone else to push him.
If I look at it that way, it seems he's rather passive compared to previous Riders.
You ever hear the saying "The road to Hell is paved with good intentions?"
More importantly, think about who's the one doing it almost every single time? Sagara. Now, think about those moments as well, notice how the tone gets progressively more sinister, going from the off-handed "You interest me" gift of the Genesis core, to Kachidoki's almost out of body experience(Everyone just disappearing, the lights going dark, kind of doubt it really happened and was probably all an illusion created by Sagara.) to finally Kiwami's down right sinister overtones.(Also... only time Kouta really made a grab for them. Every other time, Sagara either placed it on the table or forced it into his hand. Another part that makes me suspicious of Kiwami[Seriously, I'll be upset if it never comes back to bite him in the ass, you don't just make deals with the devil and come out unscathed.] and what it could bring.)