This study proposes a tractable stochastic choice model to identify motivations for prosocial behavior, and to explore alternative motivations of deliberate randomization beyond ex-ante fairness concerns. To represent social preferences, we employ an additively perturbed utility model consisting of the sum of expected utility and a nonlinear cost function, where the utility function is purely selfish while the cost function depends on social preferences. Using the cost function, we study stochastic choice patterns to distinguish between stochastic inequity-averse behavior and stochastic shame-mitigating behavior. Moreover, we discuss how our model can complement recent experimental evidence of ex-post and ex-ante fairness concerns.