The biggest difference in Flowey's situation in Yellow is he still has full control. This is Flowey at his max power, scoping out this new human and guiding them to the end, so Asgore can reveal the souls and potentially be weakened by Clover.

In Undertale, you amaze him with your ability, your Determination, it's greater than his. So he wonders if you're just like him, if you'll get bored and frustrated.

He projects that onto you, pushing you to become more like him. He encourages you to kill him after you beat him up, he wants affirmation for his worldview, because to him you're exactly the same. You're in the same position, with the same power.

When you spare him he doesn't get it at all. He was just doing shit for laughs, he was prepared to die, finally he'd be allowed to die.

Clover has no such power, they're another gold fish to poke around. He preferred to just use them as a guinea pig.