I can't understand how people can come up with a cat or dog journal/blog,
to write like they themselves are the dog and have feelings and sentiments towards everything,

Yes la the animals do have feelings but maybe not the ones you are assuming.

It goes typically like

"Yay, today Mummy is bringing me to xxx's house. I can get to see my friends bobo and lulu. We are going to spend a great time playing in the garden. I love the new treats Mummy has bought for me. "

roleplay or what.