An eye 目 peering at a tree 木 — to inspect, to look at. Looking goes both ways, so 相 came to mean 'mutually'. Read xiàng, it's the look itself: appearance, photograph.
相信 'believe', 相同 'identical', 互相 'each other'. As xiàng: 照相 take a photo, 相机 camera. Old China's 宰相 (prime minister) was literally 'the one who watches over'.
An EYE watching a TREE — two things facing each other: MUTUAL.
相声 xiàngsheng, China's beloved comic crosstalk, literally means 'mutual voices' — two performers bouncing off each other.