Hola D K

The rationale for using "ser" instead of "estar" in the phrase "a veces es frustrante" lies in the nature of the adjective "frustrante".

"Ser" is used to describe inherent characteristics, general qualities, or the nature of something, which are typically seen as more permanent or defining. In this context, "es frustrante" implies that the situation is generally and inherently frustrating.

On the other hand, "estar" is used to describe temporary states or conditions, often linked to a specific moment or situation. However, in this case, the use of "estar" would sound odd because "frustrante" is considered a characteristic of the situation rather than a temporary condition.

Therefore, "a veces es frustrante" correctly uses "ser" to convey that something is inherently frustrating at times.

Hasta pronto

Silvia