Confirmed, I'm not the romantic type and I skipped many of the supposed-to-be romantic magical scenes.
Then, if ignoring the "romantic" spices from the interactions between Celia and Marco, why are they in crazy love with each other?
Perhaps. Perhaps, it's because he/she is the only other person in the world that understands and resonates with you. How lonely it would be if you are the only one that speaks the "magic" language. No matter how well you do, normal people won't be able to appreciate the smartness and elegance in your move, except the superficial appearance.
that's why Tsukiko and her competitor were also in love. It will be almost impossible not to love the only person in the world who understands you, feels what you feel, and, of course, is bounded with you.
... that is, 人生得一知己足矣。
about the ending, I'm not happy with it: Celia/Marco's 50%-real way of living not convincing in a technical way; Tsukiko's motive to sacrifice Marco for Celia not clear given that Tsukiko is the one who survived but suffered in her remaining life; the state of being bounded to the circus as half-human doesn't look like a satisfying destiny to me (come on, is love (of your partner) the only enjoyment to have for life?)
I'm too practical, I guess....