Because Sagittarius essentially spends all of their time in their own heads, they are all too familiar with the terrain of mental fortitude.
They are good at processing information, and they have a variety of coping mechanisms for dealing with stress or other problems in life.
Aries is one of the strongest signs mentally because they have a history of getting themselves into sticky situations and finding ways to survive.
As they say, Aries is a headstrong sign that tends to stand strong because of sheer nerve; they will survive and flourish because that is how Aries thinks.
This isn't a joke, to be honest. Consider the job finished if Leo puts his or her mind to it. Leo has extraordinary mental fortitude and their prowess will endure the test of time.
The vast majority of the amazing things accomplished in life by those born under this sign are mental, as opposed to physical.
To be that sadistic, you really have to have a strong mind.
Furthermore, Scorpios are the true sadists of the zodiac. It's just a fact that they genuinely take pleasure in your suffering.
Even though they aren't always the top "go-to" option for mental health, they still stand a good chance of taking home the prize thanks to their mental fortitude. Health? Nah. Strength? Yah.
Taurus has a strong mind and is prepared for the world; it is a determined sign.
Cancers have a tough time being pegged as the weak homebody who bemoans their terrible fate all the time.
So, to counter the effect of the entire world just assuming their frail basket cases, they have honed their "mentals" into one big statement of mental strength.