flag, praise
旌 centers on a flag or banner used to signal recognition or honor, which naturally extends to the idea of praise or commendation.
旌 combines 方 (a flag or banner) with 生, which likely contributes a phonetic element. The character originally referred to a type of banner or pennant used in ceremonies or to signal commendation.
The left side 方 is a flag, and the right side 生 suggests life or vitality. Imagine a vibrant flag raised high to praise someone's achievements.
For セイ, picture a flag being raised to say 'say' something good about someone: say -> セイ, and the flag waves in praise.