I've very recently started to view things in a pantheistic/panentheistic manner after being agnostic atheist for the majority of my life (currently 22, raised Jewish). The reason, for me, that humanism meshes with the idea of pantheism is that of a self-correcting system. There is a will in all things to evolve. To improve, to become better versions of ourselves (our Ideal selves as William James would put it). Plants, animals, humans, all behaving in a manner towards the maximal (with varying degrees of effectiveness). To behave in a manner which may harm another, is also to behave in a manner which may harm the self. If I consider myself anything other than everything, I work against myself.
Alternatively, if I'm wrong about there being a Godhead, or a source of all that is, I'll have set up conditions for others to behave as benevolent as possible without a sort of intrinsic, restrictive ideology. without a need for edifying persons based solely on my belief system. It's a win-win in my opinion :)