Gross income... Take $10k per person tax and you have $1600 pw or $6,900 pm. less say $3,500 pm living expenses for a family with 2 kids & the aforementioned mortgage payment of $2,200 and you have a $1,200 pm surplus, while rates are sitting at ~4%. Bump the rates to 6% and that surplus is reduced to ~$400 pm. That's only one unexpected bill away from $0. Not to mention this doesn't take into account car finance, personal loans, credit cards etc etc etc.