Mortar is made up of sand, lime and Portland cement.  Mortar has three primary functions; bonding, acting as the sacrificial element, and water permeability.  Mortar is what bonds (holds) the masonry units together, whether stone, brick or block.  It is also the sacrificial element of the wall meaning that if the wall moves it is the mortar that will give and allow for the movement.  Mortars that are too hard for the application are very common.  The result is that the mortar does not allow movement, so, when there is movement the masonry unit, let’s call it a brick, will crack and spall instead of the mortar absorbing the movement.  Another major issue with hard mortars is that they lack water permeability (mortars third main function) and trap water in the wall.  Moisture will always be present in masonry.  It will enter the wall or it will be present behind the wall.   When the mortar is harder than the unit laid, in our example the brick, water will move in and out of the wall primarily through the brick instead of through the mortar.  Water will freeze and thaw over the winter and destroy the brick.  Mortars should always be softer than the masonry unit used and lime based mortars usually best.  Soft lime based mortars are capable of self healing small cracks and also allow for better water permeability.  Always understand what mortar is right before doing the job.