If you live paycheck to paycheck, that means that you can eat cheap food and ideally pay your bills. If something breaks, it will stay broken. If food prices go up, you'll get hungry. If you cannot pay rent, you'll become homeless in certain parts of the world. It gets worse if you have kids.
The result: constant stress which will remain with you for the rest of your life, even if your situation gets better.
If you can read Dutch, try to find the book by the Flemish journalist Tim 's-Jongers (apologies if I spelled his name wrong). He wrote a very poignant book on how it is to be poor and what it did to his mental state, even if he has a stable income at the moment. I am not sure whether that book was translated into English.
I once knew a woman who only noticed she was pregnant when she gave birth. One night she had some belly ache. Next thing she knows, she was suddenly a mother. In her own words: she was pretty sturdily built.