The most common reason a toilet leaks is a bad or improperly installed wax ring.
The photo is of a sad story. The house was just remodeled and sold, and the new homeowner inherited new and old plumbing problems. The one we will discuss here, is improper wax ring installation. The gap between the surface of the tile where the toilet is installed and the flange to the drain is too large. In the photo you can see that that the wax ring is just about flush with the tile. The wax needs to be protruding above the surface of the tile, allowing the weight of the toilet pressing against the wax the create the seal.
Shimming the flange is an easy and relatively inexpensive fix. Unfortunately, problems like this often aren’t found until they ruin drywall ceilings or subfloors. The picture below shows the shimmed flange with wax ring protruding above the tile the way that it should be.