The heart radical at the bottom carries the meaning: thinking, wanting, and missing are all matters of the heart in classical Chinese psychology. This is what makes 想 belong to the family of mental/emotional verbs alongside 念, 思, 感.
Top component supplies the sound: 相 → 想 with tone shift. Originally 相 meant 'looking at a tree with the eye' (木 + 目), so it also adds a faint semantic flavour of mental imagery - picturing something in the mind's eye.
