It’s very easy,just grab any English grammar book,copy all the nice adjectives or mug them up and next time when she ask this question just randomly throw this adjectives,like beautiful,smart,etc… She will be happy and you will be happy..
BOOM.. Crisis solved.. 😀
I was just kidding.
First,I do not know anything about your girlfriend so I cannot tell you what to say.
But I will tell you what I reply to my girlfriend.
Honestly,I do not know the exact reason why I love her,I just do,maybe that is what true love is about,loving without any reason. But a little appreciation of the charming qualities your girlfriend have now and then just to let her know what makes her more special or lovable is always welcome.
For that you need to know your girl really well and try to understand what she feels.
Never fail or never let an opportunity to pass by of appreciating your loved one.
Thank you.Hope my answer helped.