相 'appearance' sitting on 心 'heart': holding someone's image in your heart. That is thinking of them, wanting them near, missing them — all three meanings in one picture.
我想你 'I miss you' is the classic long-distance text. 想 also powers plans (我想去 I want to go) and thought itself (想法 idea). Desire, memory and reasoning share one heart-character.
An IMAGE resting on your HEART — THINKING of it, WANTING it, MISSING it.
想 vs 要: 我想去 'I'd like to go' is soft; 我要去 'I'm going' is decided. One character changes your commitment level.