envious, be jealous, covet
羨 centers on wanting what someone else has: feeling envious, jealous, or coveting something desirable.
羨 combines 羊 (sheep) with 㳄 (a variant of 次, meaning 'next' or 'second'), suggesting the desire for something secondary or the drooling over something desirable like a sheep. The exact historical development is uncertain, but the idea of coveting or envying is central.
The top part looks like a sheep (羊) and the bottom like a person drooling (㳄). Imagine someone staring at a neighbor's sheep, mouth watering with envy.
For セン, picture a person so envious they send (セン) a thousand (千, セン) longing glances at the sheep. Send -> セン, and the envy is palpable.