if we decrease the thickness of the lens why does the focal length increase?

When we take a thick lens the light travels a longer path inside the lens compared to a thinner lens. Then when it comes out of the next surface it cuts the principle axis at a shorter distance. As because focal length is defined as the distance between the principal focus and the pole of the lens, rays meeting the principal axis at a shorter distance decreases the focal length of the lens  (principal focus is defined as the point where the parallel rays meet after getting refracted from the lens). This depends upon the thick lens maker equation about which you will learn in higher classes.