It is easier for European countries to be generous with social programs when the USA is providing their defense umbrella. If they had to defend themselves, most would have to spend several percent of GDP to have their defenses be more resilient than a box of tissue paper. With the US defense umbrella, they instead get to spend this on their social programs. It is in effect a transfer from US taxpayers to European social program recipients.
And before anyone says it, I realize that Sweden and Finland are not NATO partners, but they have separate agreements with the USA as well.