Nail polish almost always fails at the tip before anywhere else. The pattern follows from where the film is stressed and from what is underneath it.
The free edge takes all the contact
The tip of a nail contacts keyboards, zips, packaging and every surface a hand touches, while the body of the nail is largely protected.
Each contact applies a small shear force at the polish edge, and the film is thinnest and least supported exactly there.
Which is why the chip starts as a fine crack at the edge and then propagates backward across the nail as the bond continues to fail.
Nails flex more than the film does
A nail plate bends under load. Polish is a comparatively rigid film and cannot flex to the same degree without cracking.
The difference in flexibility concentrates stress at the interface, and the free edge is where bending is greatest.
Sealing the polish over the tip, drawing the brush across the edge, links the top and colour layers around the vulnerable point and slows the failure.
Oils prevent the bond forming
Nail plates carry natural oils, and any residue of hand cream or cuticle oil prevents polish adhering properly.
A film that never bonded fully looks identical when dry, and fails within a day or two under normal use.
This is what dehydrating the nail before a base coat achieves, and it is the single largest controllable factor in how long a manicure lasts.
Water swells the nail underneath
Keratin absorbs water and expands. Washing, bathing and swimming all cause the nail to swell slightly and then contract as it dries.
The polish film does not move with it, so every wetting and drying cycle works the bond loose from behind.
Gel systems resist this better because they are cured into a harder, less permeable film, though they trade that durability for a removal process that is harsher on the nail.
Thickness works against durability
Thick coats trap solvent, take far longer to harden fully and stay soft under a dry-looking surface for hours.
A soft film dents rather than resisting, and the dents become the starting points for later chipping.
Thin coats allowed to dry properly produce a harder, better-bonded film, which is why application method usually matters more than the price of the polish.