Maybe it's the cynicism that pervades so much post-modern art. Maybe it's the 'world out there' being defined by its fiscal gloom and the glaring failures of capitalism.
But when you walk into Yoko Ono's new exhibition To The Light at Hyde Park's Serpentine Gallery and see three identical mounds of earth labeled 'Country A' 'Country B' and 'Country C' in front of the classic 'War Is Over' poster she made with John Lennon, the wave of hippie idealism that hits you feels more refreshing than any old cliché has any right to.
Yoko Ono is an artist who wants to make you smile - quite literally in the case of her crowd-sourcing #smilesfilm project, that encourages users around the world to contribute footage of themselves smiling for her growing global database of grins.
In ten or so upturned World War II helmets suspended from the ceiling, she's even left a gift for anyone who visits - one piece of a jigsaw. The idea is that we might all one day come together and build the picture: of a beautiful blue sky.
Aaah. Is it wrong that this sort of mushy, flower-headed sentiment feels so good? Or to put it another way - like such a reprieve?
Amaze, by Yoko Ono (1971)
On you go through the brilliantly white rooms, the odd soundtrack of a hawk's squawk and human heartbeat repeating on a loop from above. Amaze - a maze of Perspex first made in 1971 - encourages visitors to navigate a route to its centre where a surprise I won't spoil here awaits you. One clue though: it isn't a diamond-encrusted skull.
Elsewhere, a new piece made specifically for the show demonstrates Ono's knack for making use of her environment.
Two perfect white tables sit looking out onto Hyde Park with a pad and some pieces of paper. Where Do You Go From Here? she asks, encouraging visitors to jot down their hopes for the future and stuff them in a box.
It's simple stuff - no different really from her Wish Trees from 1996 (also present) - but sat in any other gallery in London it wouldn't work. After all, why else do we go walking through parks if not to find inspirational and plot our futures?
The ghost of Lennon is a guest, but not an overbearing one. One of six videos on a wall includes her 1968 slow-motion of him smiling. There's an almost eerily prophetic, Jesus-like quality to the composition of the shot.
More touching still is John Plus Me, a sheet of paper containing their footprints. A caption explains how they had fun making footprints when Lennon was alive. Now, Ono positions them walking upwards, towards heaven - the direction she now 'realises he was heading in all the while'.
There is a clearness of thought and cleanness of execution running through Yoko Ono's work that reminds you how thrilling and fun conceptual art can be.
Film No. 5 (Smile) by Yoko Ono (1968)
Another highlight is her famous Cut Piece performance pieces, the 1964 original and a 2003 version, positioned opposite each other in the same room.
In it Ono sits still on a stage wearing a long gown and invites members of the audience to come up one by one to cut away a piece of her clothing until she's naked.
There's a weird tension to it, more palpable when she's a nervous-looking 31-year-old than a composed woman of 70. But the piece is all about the audience - how much they dare remove, how they conduct themselves on stage.
It encapsulates that other rare quality Ono has. She's an artist who wants you to be an artist too. She doesn't want there to be any barriers between her and you, any top-down, artist-to-the-masses didacticism.
That was never her bag, and it's today, in the age of the amateur - when we're all artists and photographers and journalists, if we want to be - that you understand just how ahead of her time she was.
A much-derided figure, then, espousing a much-derided message of love and peace and all that hippie stuff. But at the age of 79, Yoko Ono is far from an irrelevance or a spent force.
She's an artist who wants to make you smile, and so much of what's on display here does that with an ease and precision that shames artists half her age.
Yoko Ono Serpentine Gallery Exhibition
Yoko Ono To The Light will run, free of charge, at Serpentine Gallery until 9 September 2012.