To simplify this expression, we first expand the numerator of the fraction:

(a+b)^2 = a^2 + 2ab + b^2

Now, we substitute this in the numerator of the fraction:

a^2 + 2ab + b^2 / a^3 - a^2b - ab^2 + b^3

Simplify this further by factoring out common terms:

(a^2 + 2ab + b^2) / (a^3 - ab^2 - a^2b + b^3)

Now, we need to factor the denominator:

a^3 - ab^2 - a^2b + b^3 = a(a^2 - ab - b^2) - b(a^2 - ab - b^2)
= (a-b)(a^2 - ab - b^2)

Substitute this back in:

(a^2 + 2ab + b^2) / (a-b)(a^2 - ab - b^2)

Now, we can't simplify this any further since there are no common factors in the numerator and denominator. So, the final simplified form of the expression is:

(a^2 + 2ab + b^2) / (a-b)(a^2 - ab - b^2)