A clear understanding of the temperature evolution of the longitudinal spin Seebeck effect (LSSE) in the classic Pt/yttrium iron garnet (YIG) system and its association with magnetic anisotropy is essential towards optimization of its spin-caloric functionality for spintronics applications. We report here for the first time the temperature dependences of LSSE voltage (V LSSE), magnetocrystalline anisotropy field (H K) and surface perpendicular magnetic anisotropy field (H KS) in the same Pt/YIG system. We show that on lowering temperature, the sharp drop in V LSSE and the sudden increases in H K and H KS at ~175 K are associated with the spin reorientation due to single ion anisotropy of Fe2+ ions. The V LSSE peak at ~75 K is attributed to the H KS and M S (saturation magnetization) whose peaks also occur at the same temperature. The effects of surface and bulk magnetic anisotropies are corroborated with those of thermally excited magnon number and magnon propagation length to satisfactorily explain the temperature dependence of LSSE in the Pt/YIG system. Our study also emphasizes the important roles of bulk and surface anisotropies in the LSSE in YIG and paves a new pathway for developing novel spin-caloric materials.