Why are the two lobes of p orbital given positive and negative sign?

The answer of this question lies in quantum mechanical theory proposed by Erwin Schrodinger. According to the theory an electron in an atom can be described by a differential equation whose particular solutions are the atomic orbitals . These orbitals are represented by psi(greek) whose solution in the polar co-ordinates generate the shape of the orbital . For p orbital the shape is dumbell with one lobe - & one +. These are the signs of psi function & don't have any physical significance. Don't confuse it with charges.