Being young, no kids, girlfriend but not fiance/wife yet I'd say it can go a long way to 'buying' happiness.
Apparently, the way to be happy with money is experiences over actual possessions.
If i had money I'd spend it on a mix of both. For example, instead of spending a day off playing games on my PC, i'd have a purpose-bought track motorbike, which I'd take on track days.
Would that make me happier than sitting on a PC? I'd expect so
Equally, I also strongly agree that money isn't everything. If someone was fundamentally unhappy, if they won the lottery, I expect they would still be unhappy, unless they were originally unhappy because of money stress/problems.
A track bike needn't be expensive, you may even be able to afford one now