live, dwell
棲 centers on making a place one's home: living somewhere or dwelling in a location.
棲 combines 木 (tree) with 妻, which likely contributes the sound and the idea of settling down, suggesting dwelling among trees or making a home.
A tree 木 beside a wife 妻: picture a couple building a home together under a tree, living and dwelling there.
For セイ, imagine a couple saying 'say' their vows under the tree where they will dwell: say -> セイ.
cohabitation (esp. of an unmarried couple); living together
to live (of animals); to inhabit; to nest
amphibious animal
almost living together; spending most nights with one's partner
living a quiet life in seclusion away from the masses
old home; former home; old nest
habitat isolation
living in tranquility; retiring to the countryside
the morally upright have a hard time making friends; fish will not live in water that is (too) clean
herpetology
one's final abode
a wise retainer chooses a great lord to serve; a wise bird chooses a good tree in which to roost
secluded life