Well to be honest we also compare Thylane to Laetitia because they are both French. Besides L.Casta is the most famous French model.
Also Casta's career is amazing and although Thylane is still really young she also has a nice career and a bright future. Casta is also an actress now and Thylane seems to care about acting too.
Concerning the way they look, they both have long hair, light eyes and natural beauty.