Many “fact accounts” attribute the following quote to Mark Twain:
The more I learn about people, the more I like my dog
The quote often includes the following photo of Mark Twain and a dog.
According to WikiQuote, this quote falls under the category of unsourced/possible fakes.
I also posted this to Metabunk and can confirm that Mark Twain never said or wrote that.
Further reading: 10 things Mark Twain didn’t really say (by Mental Floss).