Budgie sound & behavior questions?

I had my budgie for a little over a week now and I've been curious about some of his sounds and behaviors. Please try to anwer as many as you can as best as you can. Thank you~

1. When I tickle Romi's chin, he makes this teeny little squack/sqeak sound and starts nibbling on my hands. He makes the sound once. Why does he make that sound?

2. When I say "I love you!" to him, softly, he chirps back REALLY softly. like. once. Why is he chirping softly and not loudly like he usually does when he hears other birds?

3. Romi doesn't bite as hard as other budgies do. Even when he's scared or angry or annoyed. He's never bitten hard enough to hurt anyone before. He doesn't destroy his toys either like other budgies. He eats fine, and he shreds paper to pieces, though... but why isn't he biting people HARD? Even when I first got him, he never bit my hard.

4. Romi doesn't fly much. His wings are clipped, but he doesn't try to fly unless he has to. He's also very still. If I leave him on my desk, go take a shower and come back, he's at the same exact place and he hasn't moved a little. He loves to play on my keyboard when I'm watching TV, though.

Thanks for answering!!