Well it has nothing to do with the Higgs, but it is due to some deep facts in special relativity and quantum mechanics that are known about. Unfortunately I don't know how to make the explanation really simple apart from relating some more basic facts. read more
This is not exactly a "why" question. We observe in nature that electric charge can come in varying quantities, both positive and negative; and we observe that like charges repel, whereas opposite charges attract. Therefore we must construct a theory which can describe this behaviour. read more
So for spin $0$ you get a $-$: like charges attract. For spin $1$ you get a $+$: like charges repel! And for spin $2$ you get a $-$ again: like charges attract. read more