We get called to fix a lot of patios we didn't build. The pattern is almost always the same: a corner has sunk, a few pavers have lifted, and water pools against the house. When we pull a paver up, there's two inches of sand on top of dirt. That's the whole problem.
Western Washington soil holds water, and water that freezes and thaws moves whatever sits on it. A patio that lasts needs to sit on a base that drains: six to eight inches of crushed rock, laid in lifts and compacted with a plate compactor each time, then an inch of screeded bedding sand. On clay soil, we go deeper and add fabric to keep the rock from sinking into the mud.
Pitch matters too. The surface should fall about a quarter inch per foot away from the house, so rain runs to the lawn instead of the foundation. It's not visible when the patio is done, but you'll notice it the first November.
After the pavers are laid and cut, edge restraint keeps the field from spreading, and polymeric sand locks the joints so weeds and ants can't get in. None of this is expensive compared to the pavers themselves, but it's the difference between a patio that looks the same in ten years and one we get called to rebuild.