Question

Why stigma at the top of the pistil is often sticky

Answer 1

It needs to trap pollen.

Answer 2

The stigma is the part of the female reproductive part of the flower. This is used on the top of pistil and it is naturally sticky by both trap pollen.

When the pollen lands on the stigma, it will grow a tube down style and into the ovary of the pistil. It is to be pollinated and from a plant stamen to transfer the plants stigma.