why not "a mi"
Simply does not work like that. "A" usually indicates motion of some sort.
Would estar work here? When I checked ser is for Relationships, Family etc. but Estar is for Emotion (from the famous DOCTOR PLACE mnemonic...)
I don't think so, no. They use ser here because it's a general statement.
We are not talking about friendship but rather an essential characteristic of that friendship, namely that it is important to me.
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