Prototypical 3D topological insulators of the Bi2Se3 family provide a beautiful example of the appearance of the surface states inside the bulk bandgap caused by spin-orbit coupling-induced topology. The surface states are protected against backscattering by time reversal symmetry, and exhibit spin-momentum locking whereby the electron spin is polarized perpendicular to the momentum, typically in the plane of the surface. In contrast, graphene is a prototypical 2D material, with negligible spin-orbit coupling. When graphene is placed on the surface of a topological insulator, giant spin-orbit coupling is induced by the proximity effect, enabling interesting novel electronic properties of its Dirac electrons. A detailed theoretical study of the proximity effects of monolayer graphene and topological insulators Bi2Se3, Bi2Te3, and Sb2Te3 is presented, and the appearance of the qualitatively new spin-orbit splittings, well described by a phenomenological Hamiltonian, is elucidated by analyzing the orbital decomposition of the involved band structures. This should be useful for building microscopic models of the proximity effects between the surfaces of the topological insulators and graphene.