I don't remember much of how I fell in love with Shiki––or really anything else for that matter––but I'm not really all that worried about it. I love her and that's all that matters. Shiki was not someone who caught my attention at first, but underneath that spunky exterior hides a troubled girl. I think because of that I found myself able to relate to her in perhaps more ways than one.
There are things about ourselves we all wish were different––I struggle with some of these myself. It can be a physical attribute, such as hair color or height, or a skill one may have that another may not, like drawing or singing. For me I sometimes wish I had a brighter hair color or could draw as good as some people. Some things I can't change (or would take a ridiculous amount of money) and others take time and practice, but I'm okay with it. I can accept me for me, even if there are things I don't like about myself. There is no one else I'd rather be.
The same can be said for Shiki as well. Shiki has an inferiority complex when it comes to Eri and wants to be exactly like her. Eri is popular and outgoing, and is good at designing clothes. Shiki is none of that. While Shiki has trouble accepting this at first, it's her friends who make her realize that she is perfect the way she is and that they love her for that. I also love Shiki for who she is too (why else would I make a shrine to her?), and that she is eventually able to see that as well makes me love her even more.