There are a number of very good sites that explain the physics of lightning. There is no direct path to any specific object and no guarantee that an object higher than another will be the point of discharge. Air is a dielectric, meaning that it provides a degree of insulation between charges of two different polarities. Cloud bottoms, particular in storms, are negatively charged, mainly because an electron is stripped from atoms of moisture as they rise. Cloud tops become positively charged. As the lower cloud builds up negative charge, the electrons on the earth's surface (and any object) are forced downward, making the earth surface (and objects) positively charged. As the difference in charges or difference of potential builds up, the air dielectric becomes less of an insulator and it will tend to breakdown, but not in a straight line, resulting in ionization of the air. This can occur anywhere depending on exacty how high the differential buildup. A leader from the cloud goes downward while a streamer goes upward from the ground or objects on the ground. The result is a complete breakdown and a conductive path is established. Positive electrons like to collect on very narrow pointed objects, hence the greater probability that a conductive path is greater from those objects to the downward leader, i.e. lightning rods, towers, etc. But a person standing on the ground may very well provide that high collection point. A lot of people are injured or killed by lightning when the lightning strikes a nearby object. Secondary paths are opened up as well and discharge occurs simultaneously with the main strike.