Checking the Cessna 152 POH it says to increase takeoff distance by 15% of ground roll (makes sense due to increased friction). Also says to increase landing distance by 45% (doesn't seem to make sense). Would like to understand why.
The long and short of it is that wheels are designed for smooth tarmac. While grass increases friction on the wheels on takeoff (making the aircraft work harder to get up to speed), it also reduces the efficiency of the brakes, thus taking longer to slow down.
And to add to @GdD's point, grass retains moisture much better than tarmac, so in wet conditions it is considerably slippier (and will remain so for longer afterwards).