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).
HoaxEye 
People attribute so many quotes to Mark Twain. It’s like if they don’t know who said it, they just attribute it to him. This site is awesome! I love your passion for truth. Keep up the great work!
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