Flagstone on dirt

Hi,
I had a flagstone patio laid a couple years ago. The landscaper laid it on dirt, no sand or gravel.
This year, we had to do some other work around a pool and driving over the patio with a skidloader broke some pieces and compacted others. Now we think we should pull up the patio and re lay it. My question is, is it proper to lay the flagstone on just dirt? It looked fine, the landscaper who did it filled the grout lines with pea gravel and river rock. It looked really great and no grass/weeds grew BUT it does hurt your feet walking on it (the pea gravel of course does not stay in the grout lines) so I am thinking we should re think this.


Our dirt is clay. Its very compacted where the patio is.
My hubby and I are thinking about doing this ourselves. We also think we should lay it in sand this time and fill the grout lines with proper filler thus eliminating the ouch in walking on it.

This is my third patio in my 15 year old house!
The first was laid with grass as the grout lines. That was hard to keep looking nice.
The next was we took out the grass and put coco mulch in between, but still messy.
The last was the pea gravel and river rock....again looks cool but impractical.

I think this should truly be the LAST time I do this!