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.
[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.)