It isn't necessarily that money buys happiness, but it can help pay for experiences which in turn lend themself to happiness. Some of my best memories are on expensive holidays with a bunch of friends doing crazy shit like renting small boats and motorcycles on the same day etc. It wasn't that those items brought happiness, but they helped create an experience that is much more memorable than say, sitting in my friend's living room.
I think regret is totally subjective. I don't want my last meal before I get some horrible disease to be something healthy and low in calories, yet there are people out there who would have no regrets because being a super fit and healthy human year round is what they enjoy in life.
Of course there are total idiots who buy the newest phone, newest clothes, newest 'x'...but splurging isn't always the stupid thing to do if the intentions are good