too much of anything is bad for life. Sugar water has much the same problem, or extremely salty water. The problem is osmotic pressure.
Roughly, the natural state of things is spread evenly, so all that sugar outside the cell finds its way inside (unless the cell pumps it out). But too much inside the cell can disrupt the other processes of the cell, or even rupture the cell walls.
Iirc,Osmosis, with water rather than sugar, is also the effect that makes your hands prune when they're in water for too long.