Quote from FB123:
Actually, I think I know what it is. Gates is a shy, quiet, withdrawn, internally focused person. His strength is in seeing the big picture of things, and understanding where supply/demand for technology is going to go in an aggregate way... but what he is NOT experienced at, and certainly wasn't a few decades ago, is in reading and understanding people on a personal level. To a quiet nerd like Gates, a brash loud talker like Ballmer might seem appealing: "Here's a guy who is good at all the things I'm am not good at: talking to people, convincing them to buy our product and/or work with us, etc... I'll just hire him and let him handle all that stuff so I can concentrate on what's important to me!". I've seen this before, where a shy/quiet entrepreneur type gets a salesman-type of partner because he thinks it's going to compensate for all of his weaknesses. Usually it doesn't work out, as the loud idiot ends up causing a lot more trouble than he's worth... but the Gates type will often stick loyally with him for a long time, and because of the personality difference will often not have the balls to challenge and/or get rid of him, unless the guy REALLY screws up. Ballmer might very well be an intimidating type of person to someone like Gates.
Einstein was a genius too, but wasn't the smartest guy in his personal life... just because you're smart in one area, doesn't mean you're smart in all areas.