A word/phrase for someone who finds significance in anything/tries to rationalise anything

A predestinarian believes in predestination.
In your friend's case it is a belief in fate/destiny rather than in anything theological.

Cambridge dictionary definition predestination:
the belief that people have no control over events because these things are controlled by God or by fate.

In a philosophical sense your friend might also be a determinist of one form or another.

Wikipedia has an article on determinism with the following broad definition.

Determinism is the philosophical belief that all events are determined completely by previously existing causes.

He might for instance believe in predeterminism. (On possible subtle differences between predeterminism and predestination consult your local philosopher/theologist or Google.)

Predeterminism is the idea that all events are determined in advance. Predeterminism is the philosophy that all events of history, past, present and future, have been already decided or are already known (by God, fate, or some other force), including human actions.