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Please explain my question like I'm 5 too? ELI5: Why do ships have circular windows instead of square ones? : explainlikeimfive
The real answer is that shapes with sharp corners are structurally weak. Arcs and circles are very strong shapes. If port holes were squares, the openings would get damaged and worn out sooner.
Stress is a physical quantity that averages over internal forces (e.g. van der Waals forces, Ionic bonding, collisions, ...) that neighbouring atoms/molecules of a material exert on each other.
If a force acts on an object, it causes stress inside the material. Stress behaves like a fluid in the sense that stress flows trough the material. Now, if there is a sharp corner, stress can't flow around it which causes a concentration of stress in that point.
If the stress in that region is to high, the material will fail. Therfore having round corners instead of sharp corners can make a structure stronger. The following picture from Wikipedia illustrates this quite well:
