is a commonly used expression which indicates that ‘(A) is natural’, or ‘(A) is a matter of course’. This is a combination of the て-form of either verbs of い-Adjectives (or で when used with nouns), the noun 当然（とうぜん） ‘naturally’, and だ.
It is only natural that a cocky person like that is hated.
I am the one that made this soup so, of course it is delicious.
This building was only built last month, so of course it is beautiful.
is a fairly strong phrase, and will often be used when pointing out things that the speaker considers to be obvious facts.