Translating song lyrics: SHE Starlight 2 verses
Missing or omitted:

Line 1: "也許是你笑的弧度和我(的)很像 "

**the possessive marker 的 is omitted, 

我= I /  我的 (with object)= my / 我的 (without object)= mine

Line 2: "(我們)才會背靠著背一起等天光"

** The subject pronoun "我們"(us) is omitted, 
In Chinese, we omit pronoun whenever possible. 

Break down:

[1.也許是] [2. 你笑的弧度] [3. 和我(的)很像]

[1.may be it is because] [2. the curvature of your (mouth) (when you) smile] [3.is very similar to mine ]

** they simile like each other, imply their personalities are similar too.

[4.(我們) 才會] [5.背靠著背] [6.一起] [7.等天光]

[4. that made us] [ 5.back pressing against each others back] [6.together] [7.waiting for dawn]

** It is a graphic description of a couple sit closely together in darkness and just let time pass. (supposedly a romantic scene.)