Question

Why would auxin make the plant grow towards the sun light?

Answer 1

It is because of the dormation of auxin on either side of plant which bent the plant towards the sun and even because
of the the light of sun

Answer 2

Auxin term is applied to the indole3 acetic acid, and to other natural and synthetic compounds having certain growth regulating properties.They are generally produced by the growing apices of the stems and roots from where they migrate to the regions of their action.
in presence of sunlight they get concentrated in the shady portion of the tip resulting in bending of the shoot towards sunlight..

hope this helps u...