How do I say ‘I miss you’?

The final 了 makes the feeling sound current or newly noticeable, which often adds warmth.

想 is one of Chinese’s high-leverage verbs, covering think, want, and miss depending on the object and context. 我想你 leans intimate in adult speech — it is natural between partners, family, and very close friends, but can feel too direct in a casual acquaintance. The final 了 matters: it marks the feeling as current and newly noticeable (not a general fact about you), which adds emotional warmth. If you want to express missing someone without the emotional directness, 好久没见了 (“haven’t seen you in ages”) is the safer warmer.