Warum stellst Du mir all diese Fragen.

English Translation

Why are you asking me all these questions?

Why is “Du” in uppercase?

It has no meaning. Some Germans simply consider it more polite or respectful or appreciative to write words like “du” or “euch” with a capital first letter. Like, if you’re too familiar with each other to use the polite but distant “Sie” but still want to express your respect. But that’s not a norm, it’s not disrespectful to write these words all lower-case.