I write using uppercase letters at the start of sentences because:

1. It’s just how I want to.

2. It’s the official way to do it.

3. It makes me have a minimum amount of personality on the internet that I can control more easily.

However,

1. I don’t start a sentence with an uppercase letter if it’s with a proper noun that’s lowercase (yes that’s a thing apparently).

2. I rarely start a sentence with a lowercase letter depending on my mood. But that’s just when I’m extremely depressed (which doesn’t happen that much thankfully).