envy, jealousy, doubt
猜 centers on a suspicious, uneasy feeling toward others: envy, jealousy, or doubt that makes you question someone's motives or worth.
猜 combines the beast radical 犭 with 青 (blue/green). The exact historical development is uncertain, but the character likely expresses a negative, animal-like emotion associated with suspicion or envy.
A beast (犭) turning green (青) with envy: imagine a jealous animal whose fur takes on a sickly green hue as it eyes what others have.
For サイ, picture a jealous sigh (サイ) escaping from the green beast: sigh -> サイ, as it watches others with envy and doubt.