I completely disagree with a previous answer given by someone. There is a famous saying in brick masonry that “Mortar must never be stronger than bricks.” It is crucial for the bond to be weaker than the brick from both a strength and a durability point of view. Walls where the mortar was stronger, eventually break up on their own with time without extra forces applied.

First, you must understand that nobody is trying to break your wall. The extra loads, in addition to compressive loads from the top, are only due to minor movements from temperature changes, structural settlement, etc. and lateral forces. Even though you do make a stronger mortar, breaking won’t be significantly toughened.

Now, coming to the actual reasons, the joints must be weaker because of the wall cracks or breaks, it should preferably do so trough the mortar than the bricks because mortar will be easier and cheaper to repair. Also, if bricks break, the whole wall could come down, but mortar won’t break at all places in the wall. Also, rich and strong mortars tend to be rigid against minor movements and are comparative more vulnerable to initiate cracks as a consequence.

Secondly, from a durability standpoint, it is even more important to have a weaker mortar. If there’s hard and dense mortar around porous open textured bricks, the bricks absorb moisture more readily during rains. Now when the weather improves, bricks can’t dry up freely from all sides because of hard imporous mortar surrounding. This water retained in the blocks weakens them. Further, if there’s extreme cold, freezing and thawing will cause the bricks to split and break, which can be catastrophic. If we have a weaker mortar, water will take a path, thought it.

So, you need a weaker bond and stronger brick so as to protect the bricks from failing rather than the mortar because it will be more catastrophic and expensive.