、。
He's convinced he's the center of the universe, but to everyone around him it's just plain selfishness, isn't it.
Literal
Person-himself [topic-は] only-I-alone-revered [object-を] decide-plunging-in-[state-ている] seems-[but-けども], surroundings [from-から] look-[if-ば] mere selfishness [is-だ]-[insistence-よ]-[agreement-ね].
A rich sentence featuring a 四字熟語 (four-character compound): 唯我独尊 ('only I alone am worthy of reverence'), famously attributed to the Buddha's first words after birth but used colloquially to mean 'self-important / thinking oneself supreme.' 決め込む ('to firmly decide, to be convinced of') + ている (state) + ようだ (seems) stacks to give 'seems to be fully convinced.' The second clause uses the ば conditional with から見れば ('viewed from (the surroundings)') to present an outsider's perspective. Final よね asserts-and-seeks-agreement simultaneously.