Why is Zelda so serious in Twilight Princess. When you first meet her she's quiet probably because of what happened to Hyrule. Later she is for no reason and she talks all fancy. Maybe because of her being part of the Royal Family and almost Queen Zelda. Please Discuss this!
Why wouldn't she be? Look at her situation.
Her personality is also different. She's a different Zelda, so why would she have the same personality as another? She also appears older, around early 20's. Her entire design shows us these things. Instead of colors that are more like pink or bright, her colors are a subdued purple... lavender. Her color palette is that of an older, more mature character. Similarly, her hair is also darker. Ocarina of Time!Zelda had a brighter wardrobe and light color palette (pinks, bright blues, blonde hair) because she was younger and more lighthearted (despite her situation...) compared to Twilight Princess!Zelda. Younger/less mature/naive/etc characters usually have brighter, more colorful palettes while older/more mature/wiser characters usually have darker palettes. They could have given TP!Zelda a dark purple palette, but that would have made her appear too stoic. Lavender, while a color choice for more mature characters, also symbolizes care/tenderness, which TP!Zelda displayed in her sacrificing herself to save Midna.
Twilight Princess!Zelda is... almost stoic in her personality. She's just a lot more collected, she's wiser, she's more mature. She's older. If her personality was like Ocarina of Time!Zelda's or like Tetra's or like TMC!Zelda's... it just wouldn't have fit her role in Twilight Princess. TP!Zelda's position is also a lot different. She had to surrender her kingdom and was taken prisoner.
Her situation was pretty dire. In OoT, Zelda didn't have the option to surrender. Ganondorf wasn't a character who'd even consider that- he was all about killing off just about everybody he could. In TP, Zant was willing to make a deal, even though it was in his favor. So, Zelda had a lot to worry about. Any wrong move would cause further suffering to her kingdom.
And... of course the Adult Zeldas take their situations more serious. They're adults. TMC!Zelda and even Tetra were not nearly as mature. They cared about their kingdom/duties/whatever as much as they could, but their maturity levels can only be so high if they are still kids.