Read A.Zee, Quantum Field Theory In a Nutshell, Princeton, Chapter I.5, p 30 (first edition)
In Quantum Field Theory, "forces" between 2 "charged" particles correspond to an exchange of "virtual gauge bosons". For instance, the repulsive force between 2 electrons, corresponds to an "exchange" of a "virtual photon" (a perturbation of the photon field). Here the gauge boson is the photon, of spin 1.
We may consider a graviton theory as a QFT, in this case, the charge is the momentum/energy of the particle, and the gauge boson is the graviton, of spin 2.
In these theories, you have to write a Lagrangian, and you have to respect a sign coherence about the euclidean action which has to be positive. This constraint gives you the correct sign for the Lorentz-invariant Lagrangian.
Now, you may calculate the interaction energy between 2 "charged" particles. In fact, one use currents instead of charges. So, for instance, for Quantum Eelectrodynamics, the interaction energy is a functional of currents and a gauge boson quantity named propagator.
For a graviton theory, the "current" is the stress-energy tensor, and the propagator is the graviton propagator.
The constraints about the sign of the action I speak above, have a direct consequence on the sign of the propagator. The consequence of that, is that particles with same charge exchanging a particle of odd spin, repeal each other, while particles with same charge exchanging a particle of even spin, attract each other.
So, for a graviton theory, with spin 2, we see that particles with same charge attract each other.