Maybe is because I'm older now, so LIs that behave like highschool childs aren't that attractive for me anymore, maybe 'cause most of the activities you can do with her (pre and post actual relationship) are things I actually experienced and enjoyed in real life.
I think she just clicks well with who I was and still am as a man, she might not be the hottest looking, the craziest one or the sweetest one, she just feels "real" and as a consequence the "right" choice.
It's hard to explain it better is just what it is, but is also credit to the author, is relatively easy to write stereotype characters that are anything but, Lexi is a stereotype sure, she just happens to be the stereotype of woman I always loved dating.