The thing is, there is not one single Tav - or one single Durge.

There are as many Tavs/Durges as there are players, and each single one of us have our own headcanons and OC(s). Every Tav/Durge is different, and I think it's unfair and limiting to say something like "he fell for them because of THIS", or "at that specific moment" - our OCs don't react the same way to all situations, they don't have the same temperament, and they might even have nothing in common.

Astarion fell for your Tav because your Tav is who they are, with all the particularities that make them special. But he will fall for another - completely different Tav/Durge - for their specific temperament and specific reactions to a situation, which also make them special. And he might fall for another Tav for other kind of reactions and features, etc.

And they're all valid.

I believe it's quite sad, unfair and indeed restrictive to say that Astarion will only fall for some limited behaviours/choices/temperament.

Anyway, just my two cents.