Stickers & labels have varying strengths of adhesive because we all use them in various applications.
The two factors that define how strong the adhesive needs to be are surface energy & how smooth the surface is.
A smooth surface is easier to stick to simply because more of the glue on your sticker touches the surface.
Surface energy (aka surface tension) can be a complex subject, but basically, it's how sticky a material is. The higher the surface energy, the easier the glue will stick. This image shows how surface energy changes between materials:
If you're ever in doubt, either ask your suppler of labels or stickers or go for a permanent adhesive (aka medium strength). It's the best all-rounder.
Some more information about surface energy
In technical jargon, surface energy is the degree to which a material surface will repel or attract.
Imagine a freshly waxed or brand new vehicle. The water beads and runs off efficiently. The wax creates a low surface-energy, so the water cannot stick to it. The technical term is called 'wetting-out.' The easier it is for the adhesive to 'wet-out,' the better the adhesion.
High surface energy values attract, whereas low surface energy values repel. Materials with higher values bond better because good wet-out occurs of the adhesive.
Wet-out is an industry term used to describe the flow of an adhesive over a material surface. Good wet-out ensures maximum contact and helps achieve a strong bond.
We measure surface energy in dynes per centimeter (Dyn/cm).
To get even deeper into this subject, Adhesives.org is a useful resource.