Learn English – Phrase for “a person who cares much more about other people’s business than their own”

It could be someone with Male Answer Syndrome....

(defined as: The tendency for some men to answer a question even when they don't know the answer.)

My humble suggestion is

He lets emotions get the better of him.

Or maybe

He fails to walk the line.

One of the definitions for "walk the line", per Wiktionary, is

(idiomatic) To behave in an authorized or socially accepted manner, especially as prescribed by law or morality; to exercise self-control.

"Walk the line" and "toe the line" are idioms more appropriate when describing a person in a subordinate position, or trying to conform to other people's wishes and/or expectations.

"Get the better of" could be more appropriate if your goal is to describe, say, a high judge in a country with powerful and independent judiciary. He wants to be impartial out of considerations of honor etc., but sometimes emotions get the better of him. After all, he is a human being.

Another one:

He fails to keep his emotions in check.

Another:

He is driven by human passions and beset with human frailties.