As to Orks I didn't think they had a 'soul' as such, they're definitely fun guys (fungi?) to be around, but no soul as such, they just regrow a new presence when they get offed. Eldar I think are just like somebody said, a mix of the time and risk to actually mate. They feel much more intense passion (in fact any feelings) more than the other races, which was why they fell so hard and 'created' slannesh.