Think of it like a wheel. If you drive on the same single wheel, exerting all of life’s pressures, the wheel will begin to wear and get divots, making it roll less smoothly. If there’s an emergency, that single wheel is compromised and can’t get you where you want to go.
If you spread out that pressure to three other wheels, like a hobby, work, friends, if any of those wheels get divots the remaining wheels can help to carry the load and get you where you need to go.
It also lets life roll along more smoothly.
Tdlr: don’t put all eggs in one basket. It’s too much pressure, even if he loves you to bits. And if something goes wrong, having other outlets will make that particular problem seem less overwhelming, more temporary, and less likely to flip you into a dark depression.