She thinks I love her as a joke. Why?

It's too early for you to draw a conclusion. Although the word "joke" makes people look uncomfortable, adding a lifetime in front of it is intriguing.

Do you think if you give it to her in front of her friends, do you think she will say something very touching and cherished in front of her friends? Put yourself in others' shoes. If a girl gives you a love letter in front of your friends, won't you be shy?

Don't you think it's too one-sided to jump to conclusions just because of the word joke? Besides, she accepted your book, and her words "in this life" and "alive" are also positive responses. Of course, there is too little information to say that she must like you, but at least, it is not enough to prove that she really takes you as a joke.