て当然（とうぜん）だ 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.

• あんな生意気（なまいき）なやつは嫌（きら）われて当然（とうぜん）だ。
  It is only natural that a cocky person like that is hated.
• このスープは俺（おれ）が作（つく）ったものだから、美味（おい）しくてて当然（とうぜん）だ。
  I am the one that made this soup so, of course it is delicious.
• このビルは先月（せんげつ）建（た）てられたばかりだから、綺麗（きれい）で当然（とうぜん）だ。
  This building was only built last month, so of course it is beautiful.

て当然（とうぜん）だ is a fairly strong phrase, and will often be used when pointing out things that the speaker considers to be obvious facts.