Why do people have to sign in to "like" a post?

Why do people have to sign in to “like” a post? One of my subscribers has repeatedly tried to like my posts and even though she does go ahead and put in her user name and password (which are correct) she is unable to like anything. 1) why make people sign in and can I disable this?
2) why can’t a subscriber who is putting in her info not like things?

website: janetmyatt.com

Thanks so much. Also, I’m not a programmer so if I need to change code I’d need idiot-proof step-by-step instructions. Sorry about that!