There is nothing "easier" in feeling unhappy or angry. Mental illness is widespread across billionaires despite their wealth. It's also well known that mental illness is less common in traditional, less developed societies.

> ... I think the attitude of always needing to be angry ... can itself tip some people into mental illness.

No. People cannot artificially create mental illness in themselves by simply thinking "negatively". There is no simple self-destruct button in our mind: people need to go through (untreated) trauma, pressure, abuse from external sources or diseases or serious genetic conditions.