enjoy, be contented, joyful
愷 centers on a state of happy satisfaction: enjoying something, feeling contented, or being joyful.
愷 combines 忄 (heart) with 豈, which likely contributes the sound and may carry a sense of pleasure or ease. The exact historical development is uncertain.
The heart radical 忄 beside 豈 suggests a heart full of easy contentment. Picture a relaxed heart enjoying a simple pleasure.
For カイ, imagine a joyful person flying a kite: kite -> カイ, and the heart feels light and content.