Tocar vs. Tocarse
 

Why is it "Ella se toca el pelo" instead of "Ella toca su pelo"?

When one is touching one's own hair, the reflexive verb (tocarse) is required.

The other sentence says that 'she' is touching someone else's hair. The possessive adjective/determiner 'su' can mean 'his'/'her' (someone other than herself)/'its'/'your' (formal). So, the other sentence is quite ambiguous without any context.